Readme: Explain using multiple email backends
Exception cleanup

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).
Raise NotSupportedByMandrillError for unsupported attachment mimetypes.
Move MANDRILL_API_URL to package root (out of backend)