[Installer] Don't generate documenation if already installed

It checks that the documenation exist for a given version of a pod
and in that case it doesn't generate it again.

lib/cocoapods/generator/documentation.rb

@@ -74,6 +74,10 @@ module Pod
         options += spec_appledoc_options
       end
 
+      def already_installed?
+        Pathname.new(File.expand_path("~/Library/Developer/Shared/Documentation/DocSets/org.cocoapods.#{name.gsub(/ /,'-')}.docset")).exist?
+      end
+
       def generate(install = false)
         options = appledoc_options
         options += ['--output', @target_path.to_s]

lib/cocoapods/installer.rb

@@ -60,8 +60,13 @@ module Pod
         end
 
         if (should_install && config.generate_docs?) || config.force_doc?
+          doc_generator = Generator::Documentation.new(pod)
+          if doc_generator.already_installed?
+            puts "Using Existing Documentation for #{pod.specification}".green if config.verbose?
+          else
             puts "Installing Documentation for #{pod.specification}".green if config.verbose?
-          Generator::Documentation.new(pod).generate(config.doc_install?)
+            doc_generator.generate(config.doc_install?)
+          end
         end
       end
     end

spec/integration_spec.rb

@@ -208,6 +208,8 @@ else
               dependency 'SSToolkit'
             end
 
+            Pod::Generator::Documentation.any_instance.stubs(:already_installed?).returns(false)
+
             installer = SpecHelper::Installer.new(podfile)
             installer.install!