Use built-in username/password support in backends.py.

Backend is no longer reading environment variables.

MEDIAFIRE_EMAIL ->
   mf://duplicity%40example.com@mediafire.com/some_path

MEDIAFIRE_PASSWORD ->
   provided via command line via get_password() or
   FTP_PASSWORD env var or
   mf://duplicity%40example.com:some%20password@mediafire.com/some_path