Fixed #13914 -- Added natural keys to User and Group models in auth contrib...

Fixed #13914 -- Added natural keys to User and Group models in auth contrib app. Thanks, jbochi and closedbracket.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17429 bcc190cf-cafb-0310-a4f2-bffc1f526a37