#20915 -- Remove django.test.client dependency on django.contrib.auth

See https://code.djangoproject.com/ticket/20915

This proof of concept simply makes Client accept a module name that should implement the generic

login()
logout()
authenticate()
get_user_module()

API today provided by d.c.auth (that servers as a frontend to the auth backends extensibility).

What do you think? Too ugly/hackish?