Fix migrations for custom user model 

This is the same approach used by django-guardian[0].

What I like about this is:
- Does not change anything in migrations for django < 1.5, eg.: keeps using frozen models, which South says is recommended.
- Does not use django.contrib.auth.get_user_model, which can cause race conditions in app loading.

What I don't like about it:
- Feels weird have frozen models for custom user, since we don't know what fields in it. But while we don't use those fields, I think is ok.
- More to changes in new migrations

[0] https://github.com/lukaszb/django-guardian/pull/116