Add xcode documentation

This pull request adds comments to the main header files exposed by OCMock.

The comments are mostly drawn from the documentation here:
http://ocmock.org/features/

The documentation uses the headerdoc style tags that are supported by Xcode. The comments will show up for the Help Popup, the Help Inspector and during code completion.

Source of syntax:
http://dadabeatnik.wordpress.com/2013/09/25/comment-docs-in-xcode-5/