Added a simple decorator defining ``get_query_set`` or ``get_queryset``
allowing to use the latter one while preserving compatibility with 1.5.

Django (1.6+) uses a metaclass to allow usage of the former name after
renaming a method (``django.utils.deprecation.RenameMethodsBase``), but
for the 2 cases in Mezzanine a decorator seems sufficient and less
intrusive.