Introduce djrill.NotSupportedByMandrillError for unsupported functionality (previously used generic ValueError).
Change to djrill.MandrillAPIError, derived from requests.HTTPError, for API error responses (previously used djrill.mail.backends.djrill.DjrillBackendHTTPError -- retained as equivalent for backwards compatibility).
* Release notes in readme
* Update example in readme
* Note deprecation of DjrillMessage class
* Longer long_description for PyPI
* Update authors
* Bump version number (setup.py and __init__.py)