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).