Updated OneToOne relationship in ApiKey model to comply with new django 1.5 custom user.

Conflicts:

	tastypie/models.py
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.
Moved Django 1.5+ custom User logic to tastypie.models

From now on, developers on tastypie should import tastypie.models.User
instead of django.contrib.auth.models.
Removed line with invalid syntax
User model reference changed to AUTH_USER_MODEL
Added unit tests for custom users.
Username field fix
Django 1.5 compatibility