Added a new Github-specific downloader that can download repositories as a gzipped tarball.

Generally speaking, this should be much faster than cloning a repository (even a shallow one)
and we can take take advantage of Github's tarball support. I've made this opt-in (using the
:download_only option) in case this breaks any existing functionality people expect (e.g. 
using submodules).

lib/cocoapods/downloader.rb

+require 'uri'
+
 module Pod
   class Downloader
     autoload :Git,        'cocoapods/downloader/git'
@@ -28,7 +30,11 @@ module Pod
     def self.for_target(target_path, options)
       options = options.dup
       if url = options.delete(:git)
-        Git.new(target_path, url, options)
+        if url.to_s =~ /github.com/
+          GitHub.new(target_path, url, options)
+        else
+          Git.new(target_path, url, options)
+        end
       elsif url = options.delete(:hg)
         Mercurial.new(target_path, url, options)
       elsif url = options.delete(:svn)

lib/cocoapods/downloader/git.rb

+require 'open-uri'
+require 'tempfile'
+require 'zlib'
+
 module Pod
   class Downloader
     class Git < Downloader
@@ -39,5 +43,46 @@ module Pod
         (target_path + '.git').rmtree
       end
     end
+    
+    class GitHub < Git
+      def download_head
+        download_only? ? download_and_extract_tarball('master') : super
+      end
+      
+      def download_tag
+        super unless download_only?
+        download_only? ? download_and_extract_tarball(options[:tag]) : super
+      end
+      
+      def download_commit
+        super unless download_only?
+        download_only? ? download_and_extract_tarball(options[:commit]) : super
+      end
+      
+      def clean
+        # no-op
+      end
+      
+      def tarball_url_for(id)
+        original_url, username, reponame = *(url.match(/[:\/](\w+)\/(\w+).git/).to_a)
+        "https://github.com/#{username}/#{reponame}/tarball/#{id}"
+      end
+      
+      private
+      
+      def download_only?
+        @options[:download_only]
+      end
+      
+      def download_and_extract_tarball(id)
+        Tempfile.open('tarball') do |tmpfile|
+          open tarball_url_for(id) do |archive|
+            tmpfile.write Zlib::GzipReader.new(archive).read
+          end
+          
+          system "tar zxf #{tmpfile.path} -C #{target_path} --strip-components 1"
+        end
+      end
+    end
   end
 end

spec/functional/downloader_spec.rb

@@ -38,6 +38,44 @@ describe "Pod::Downloader" do
     end
   end
   
+  describe "for Github repositories, with :download_only set to true" do
+    extend SpecHelper::TemporaryDirectory
+    
+    it "downloads HEAD with no other options specified" do
+      @pod.specification.stubs(:source).returns(
+        :git => "git://github.com/lukeredpath/libPusher.git", :download_only => true
+      )
+      downloader = Pod::Downloader.for_pod(@pod)
+      downloader.download
+      
+      # deliberately keep this assertion as loose as possible for now
+      (@pod.root + 'README.md').readlines[0].should =~ /libPusher/
+    end
+    
+    it "downloads a specific tag when specified" do
+      @pod.specification.stubs(:source).returns(
+        :git => "git://github.com/lukeredpath/libPusher.git", :tag => 'v1.1', :download_only => true
+      )
+      downloader = Pod::Downloader.for_pod(@pod)
+      downloader.download
+      
+      # deliberately keep this assertion as loose as possible for now
+      (@pod.root + 'libPusher.podspec').readlines.grep(/1.1/).should.not.be.empty
+    end
+    
+    it "downloads a specific commit when specified" do
+      @pod.specification.stubs(:source).returns(
+        :git => "git://github.com/lukeredpath/libPusher.git", :commit => 'eca89998d5', :download_only => true
+      )
+      downloader = Pod::Downloader.for_pod(@pod)
+      downloader.download
+      
+      # deliberately keep this assertion as loose as possible for now
+      (@pod.root + 'README.md').readlines[0].should =~ /PusherTouch/
+    end
+
+  end
+  
   describe "for Mercurial" do
     it "check's out a specific revision" do
       @pod.specification.stubs(:source).returns(

spec/unit/downloader_spec.rb

 require File.expand_path('../../spec_helper', __FILE__)
 
+def stub_pod_with_source(source_options)
+  specification = stub(
+    :part_of_other_pod? => false, 
+    :source => source_options
+  )
+  stub('pod') do
+    stubs(:root).returns(temporary_sandbox.root)
+    stubs(:specification).returns(specification)
+  end
+end
+
 describe "Pod::Downloader" do
   it "returns a git downloader with parsed options" do
     pod = Pod::LocalPod.new(fixture_spec('banana-lib/BananaLib.podspec'), temporary_sandbox)
@@ -8,4 +19,43 @@ describe "Pod::Downloader" do
     downloader.url.should == 'http://banana-corp.local/banana-lib.git'
     downloader.options.should == { :tag => 'v1.0' }
   end
+  
+  it 'returns a github downloader when the :git URL is on github' do
+    pod = Pod::LocalPod.new(fixture_spec('banana-lib/BananaLib.podspec'), temporary_sandbox)
+    pod.specification.stubs(:source).returns(:git => "git://github.com/CocoaPods/CocoaPods")
+    downloader = Pod::Downloader.for_pod(pod)
+    downloader.should.be.instance_of Pod::Downloader::GitHub
+  end
+end
+
+describe Pod::Downloader::GitHub do
+
+  it 'can convert public HTTP repository URLs to the tarball URL' do
+    downloader = Pod::Downloader.for_pod(stub_pod_with_source(
+      :git => "https://github.com/CocoaPods/CocoaPods.git"
+    ))
+    downloader.tarball_url_for('master').should == "https://github.com/CocoaPods/CocoaPods/tarball/master"
+  end
+  
+  it 'can convert private HTTP repository URLs to the tarball URL' do
+    downloader = Pod::Downloader.for_pod(stub_pod_with_source(
+      :git => "https://lukeredpath@github.com/CocoaPods/CocoaPods.git"
+    ))
+    downloader.tarball_url_for('master').should == "https://github.com/CocoaPods/CocoaPods/tarball/master"
+  end
+  
+  it 'can convert private SSH repository URLs to the tarball URL' do
+    downloader = Pod::Downloader.for_pod(stub_pod_with_source(
+      :git => "git@github.com:CocoaPods/CocoaPods.git"
+    ))
+    downloader.tarball_url_for('master').should == "https://github.com/CocoaPods/CocoaPods/tarball/master"
+  end
+  
+  it 'can convert public git protocol repository URLs to the tarball URL' do
+    downloader = Pod::Downloader.for_pod(stub_pod_with_source(
+      :git => "git://github.com/CocoaPods/CocoaPods.git"
+    ))
+    downloader.tarball_url_for('master').should == "https://github.com/CocoaPods/CocoaPods/tarball/master"
+  end
+
 end