Django 1.8 compatibility

Added get_queryset methods while also keeping the old ones.
This should still work with django 1.4 and 1.5. (Tested 1.5, worked.)
No, this shouldn't work with Django <= 1.5, as you can see here: https://github.com/django/django/blob/1.5.6/django/contrib/admin/options.py#L221.