Test to show pollution of the auth_user_model()
Move Author Managers into an abstract base class

Copying of the default manager causes the source model to become poluted.
To supply additional managers without replacing the default manager,
the Django docs recommend inheriting from an abstract base class.

https://docs.djangoproject.com/en/dev/topics/db/models/#proxy-model-managers
Correct whitespace issues flagged by flake
Merge branch 'develop' of https://github.com/acspike/django-blog-zinnia into feature/fix-user-manager-pollution
Review the test_pollution
Review the AuthorsManager
Merge branch 'feature/fix-user-manager-pollution' into develop