Pull request for #515

This is for issue #515
* @todo #115:1hr Implement the Git Data Tags API. We need to
     *  implement the following:
     *  1) Add the operations in http://developer.github.com/v3/git/tags/
     *  to the Tags interface.
     *  2) A class RtTags, which performs RESTful request operations
     *  against the Tags API.
     *  3) A class MkTags, which mocks the Tags interface.
     *  4) An interface Tag, which represents a single Tag of the
     *  Git Data API.
     *  5) RtTags, a Tag implementation which fetches and modifies
     *  Tag information according to the Tags API, and
     *  6) MkTag, which mocks a single Reference object.
     *  Don't forget to implement unit tests and integration tests.