This allows a model to be defined which is not subject to database table
creation and removal. Useful for models that sit over existing tables or
database views.

Thanks to Alexander Myodov, Wolfgang Kriesing and Ryan Kelly for the bulk of
this patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10008 bcc190cf-cafb-0310-a4f2-bffc1f526a37