Use custom User if available.

If there is a custom User available then make use of it, if not then fallback to contrib.auth.User these changes should be backwards compatible but I have not tested this. For more info see: http://procrastinatingdev.com/django/using-configurable-user-models-in-django-1-5/ and https://github.com/dcramer/django-ratings/blob/5d91c1e20ec741334a83049d00bfb8c6dbc0223b/djangoratings/models.py