Support comments attribute in all build phases, groups and targets.

Xcode 2 and 3 used to have a comments attribute almost everywhere. I have
found instances of comments in:
- PBXShellScriptBuildPhase
- PBXCopyFilesBuildPhase
- PBXNativeTarget
- PBXApplicationTarget
- PBXLibraryTarget
- PBXAggregateTarget
- PBXGroup

lib/xcodeproj/project/object/build_phase.rb

@@ -30,6 +30,12 @@ class AbstractBuildPhase < AbstractObject
         #
         attribute :run_only_for_deployment_postprocessing, String, '0'
 
+        # @return [String] Comments associated with this build phase.
+        #
+        #   This is apparently no longer used by Xcode.
+        #
+        attribute :comments, String
+
       end
 
       #-----------------------------------------------------------------------#

lib/xcodeproj/project/object/group.rb

@@ -57,6 +57,12 @@ class PBXGroup < AbstractObject
         #
         attribute :wraps_lines, String
 
+        # @return [String] Comments associated with this group.
+        #
+        #   This is apparently no longer used by Xcode.
+        #
+        attribute :comments, String
+
         #-----------------------------------------------------------------------#
 
         # @!group Helpers

lib/xcodeproj/project/object/native_target.rb

@@ -14,6 +14,12 @@ class AbstractTarget < AbstractObject
         #
         attribute :product_name, String
 
+        # @return [String] Comments associated with this target.
+        #
+        #   This is apparently no longer used by Xcode.
+        #
+        attribute :comments, String
+
         # @return [XCConfigurationList] the list of the build configurations of
         #         the target. This list commonly include two configurations
         #         `Debug` and `Release`.

spec/project/object/build_phase_spec.rb

@@ -20,6 +20,10 @@ module ProjectSpecs
       @phase.run_only_for_deployment_postprocessing.should == "0"
     end
 
+    if "is initialized with empty comments" do
+      @phase.comments.should == ""
+    end
+
     it "can add a file reference to its build files" do
       file = @project.new_file('some/file')
       @phase.add_file_reference(file)