Fixed #3557 -- Made `SlugField` inherit from `CharField` so that its `max_length` is properly set.  Removed `get_manipulator_field_objs` method since it's already provided by `CharField`.  Thanks, Russell Cloran.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6065 bcc190cf-cafb-0310-a4f2-bffc1f526a37