Work with custom user models

[Mischback]

While django-auth_enhanced is designed to work with Django's default User-model, it should work with custom user models aswell, if some minimum requirements are met.

This issue is about implementing enough abstraction to work with these custom models, aswell as checking minimum requirements of these models.

• be as abstract as possible
  • do not assume certain model fields are always present and decouple them
    • don't reference username (use get_user_model().USERNAME_FIELD)
    • don't reference email (use get_user_model().EMAIL_FIELD)
    • implement some tests using a minimal custom user model and see, if the app still works
  • check Django's documentation about a minimal user model and try to achieve the same minimum requirements
  • implement checks for these minimum requirements
    • probable required properties: is_active, is_staff, is_superuser