Added generic foreign key support to Django. Much thanks to Ian Holsman and

Luke Plant -- most of this code is theirs.  Documentation is to follow; for now
see the example/unit test.  Fixes #529.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@3134 bcc190cf-cafb-0310-a4f2-bffc1f526a37