Fixed $5457 - the auth system now delegates permission checking to auth...

Fixed $5457 - the auth system now delegates permission checking to auth backend(s). As an added bonus, the auth backends now have some unit tests! Thanks, Florian Apolloner.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6375 bcc190cf-cafb-0310-a4f2-bffc1f526a37