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).