Adds a new method to *Repository called ResetToCommit as well as constants for
the three reset modes that libgit2 currently supports. This does not need to be
limited to Commit, we actually just need something with a gitObject, which blobs
and other Objects have, they will just require different methods. I only need
to be able to reset to commits, so that's all I'm implementing right now.
Also adds a test which updates the test repository README twice and then resets
to the first commit.
Dylan Griffin