FEATURES
* Implemented Repo.__getitem__, Repo.__setitem__ and Repo.__delitem__ to
access content.
API CHANGES
* Removed Repo.set_ref, Repo.remove_ref, Repo.tags, Repo.get_refs and
Repo.heads in favor of Repo.refs, a dictionary-like object for accessing
refs.
BUG FIXES
* Removed import of 'sha' module in objects.py, which was causing
deprecation warnings on Python 2.6.