added call to authentication callback, rewrote creds extraction

[mutax]

As reported in bug #65 I was not able to see a call to my provided callback. I am using attached patch in my development branch to allow a) authentication against the default django auth with a custom use model b) authentication against a user-specific api-key instead of a password

I would be very happy if you could incorporate the patch.

[samuraisam]

Will this affect current library users who are using authentication?