Send emails in chunks. Refactor tests.

DjangoCeleryEmailBackend now queues messages in chunks of configurable
size.  This reduces SMTP connection overhead when sending lots of mails
at once, for example using Django's 'send_mass_mail' feature.

Factored out configuration into an 'AppConf' object (requires django-appconf')

Added unittests (we only had a few integration tests).

Conflicts:
	djcelery_email/backends.py
	djcelery_email/tasks.py
	test_project/tester/tests.py
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.

Also turned a few more integration tests into unit tests.

Conflicts:
	test_project/tester/tests.py