Fixed #1991 -- Changed AuthenticationForm to disallow users with...

Fixed #1991 -- Changed AuthenticationForm to disallow users with is_active=False from logging in. Thanks, dave@rightround.com and germish@gmail.com

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3058 bcc190cf-cafb-0310-a4f2-bffc1f526a37