Updated test configuration to most recent Djangos.
Added Python 3.3 to the tests matrices.

Coverage does not run for Python 3.3 tests as the
`django-coverage` test runner doesn't support it yet.
Added Python 3 classifier to setup.py
Message during FriendRequest

    - Added optional ability to store message via add_friend() rather
      than making the user update the returned model object themselves
Merge branch 'update-test-config' of https://github.com/julianwachholz/django-friendship into julianwachholz-update-test-config