Fix a wrong import and swapping out user model support for django 1.5

Import path is pretty obvious. About the User model related code: at the previous approach you may get circular import error, if your custom User model is defined at the same file, at which you call `reversion.register()`. That is because `register` imports `reversion.models`, and those call `get_user_model()`, and this function import `models.py` of the application, which contains `register` call.