Add the `license' attribute to Specification, which can be an arbitrary string. Closes #41.

lib/cocoapods/specification.rb

@@ -32,6 +32,7 @@ module Pod
     attr_accessor :homepage
     attr_accessor :description
     attr_accessor :source
+    attr_accessor :license
 
     attr_reader :version
     def version=(version)

spec/unit/specification_spec.rb

@@ -261,4 +261,9 @@ describe "A Pod::Specification, in general," do
     @spec.platform = :ios
     @spec.platform.should == :ios
   end
+
+  it "returns the license of the Pod’s code" do
+    @spec.license = 'MIT'
+    @spec.license.should == 'MIT'
+  end
 end