Fix Rugged::Repository.discover to return a Repository object.

This is Issue #270 and I ran into this a day or so ago. It seems that when it was
implemented it was just returning the location of the repository on disk. Maybe
the this was the original intention and if that's changed I can just modify the
documentation.
Use tabs instead of spaces.
Merge pull request #272 from johnbellone/issue/270

Fix Rugged::Repository.discover to return a Repository object.