v0.43

 - Added Index methods:
    + find
 - Branch -> upstream() can now set/clear the upstream reference (GH#115)
 - Reference -> lookup() used to consider a missing reference as an error.
    It now returns undef instead of die'ing. (Incompatible change) (GH#116)
 - Reference -> lookup() now tries harder to find a reference (GH#116)
 - Reflog -> entries() now return real objects (Incompatible change) (GH#117)
 - Added Repository methods (GH#118):
    + detach_head
 - Repository -> head() now accepts an optional message parameter (GH#118)