Update for Celery 3.1

This project is now ready for Celery 3.1, which has integrated Django
support - thus, there's no need for django-celery anymore.  You can
still use older Celery versions with the aid of django-celery.

"Celery" and "django-celery" have been removed from the dependency lists
in 'setup.py' and 'requirements.txt' since these now depend on your
setup (Celery 3.1 or Celery < 3.1 + django-celery).

Also turned a few more integration tests into unit tests.