Update emailusernames/backends.py

Allow for 'username' explicitly in the authenticate() method. Some authenticators (like tastypie's) expect to authenticate() using 'username'.
Update emailusernames/tests.py

Woops. Test for new behavior.
Use kwargs to keep backwards compatibility with authenticators that expect positional args.
Merge https://github.com/urbanape/django-email-as-username
Fixes to @urbanape's fix for EmailAuthBackend.authenticate().

See pull request #45.
Merge pull request #50 from ecometrica/master

Authenticate should use `username` kwarg if no email is provided.