Don't call get_user_model in models modules

When you define a foreign key or many-to-many relations to the User model, you should specify the custom model using the AUTH_USER_MODEL setting.
Using get_user_model seems to work, but can cause circular imports in some situations, depending on your import structure and ordering of INSTALLED_APPS
Missed imports
Merge pull request #26 from Alarik/master

Thanks a lot!