Version 0.3.0

Significant changes:
* Attachments
* Mandrill templates
* Better (and less-risky) bcc handling
* Python 3 support (with Django 1.5)
* Djrill-specific exceptions

Under the hood:
* Break up tests.py into a tests directory
* Move some common things out of the backend, up to the package root
* Pull entire readme into PyPI long_description, so that github master readme can diverge from released version. (No more version branches after this one.)