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

Seems that Xcode 4 doesn't remove those comments, so supporting them is nice for people with old projects being converted to CocoaPods.

A search in Google for "filetype:pbxproj comments" gives you some of this results.

Fixes CocoaPods/CocoaPods#696