Fixed #7932 -- Made it easier to use a custom User model with the admin. Added...

Fixed #7932 -- Made it easier to use a custom User model with the admin. Added add_form attribute to UserAdmin and removed hard-coded dependancies to User. Thanks ElliottM for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8280 bcc190cf-cafb-0310-a4f2-bffc1f526a37