Fixed #7621 -- Enable superclassing of the various metaclasses we use.

Also tidy up some initialisation code in django.db.models.base.ModelBase.
Thanks, Christian Tanzer.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7846 bcc190cf-cafb-0310-a4f2-bffc1f526a37