Removes hardcoded references to User

registration/models.py is now compatible with django 1.5. The tests are not updated however, as they are still quite out of date from upstream (importing function based views that no longer exist, for example).

The workflow for creating a registration profile for a custom User will be:

1. Subclass ``registration.views.RegistrationView``, and override the 'register' method. It will probably be necessary to set the subclasses' ``form_class`` to the desired form.

2. In ``register``, create a new User object, setting the ``is_active`` flag as desired. Pass the newly created user to ``RegistrationProfile.objects.create_profile`` to create a registration profile.

3. Create a new URLConf to use your new registration view.