Updated OneToOne relationship in ApiKey model to comply with new django 1.5 custom user.
Extracted user handling code to separate function.
Changed default User for get_user_model in backfill_api_keys command.
Changed default User for get_user_model in authentication module.

More precisely, in ApiKeyAuthentication and DigestAuthentication
Changed default username for USERNAME_FIELD in authentication module.

Both in ApiKeyAuthentication and DigestAuthentication.