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
Fix typo and bad test.

I should actually write and *build* the test before commiting.
Merge pull request #51 from drodriguez/legacy-xcode3-attributes

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