Django 1.6 renamed get_query_set functions to get_queryset.

After messing up my git history in #143, here is the same thing. Only, learning from my mistakes, I did not put it in master this time.

Quoting https://docs.djangoproject.com/en/dev/releases/1.6/ :
> __get_query_set and similar methods renamed to get_queryset__
> 
> Methods that return a QuerySet such as Manager.get_query_set or ModelAdmin.queryset have been renamed to get_queryset.

This leaves us with the \_get_shared_query_set, which I renamed to \_get_shared_queryset for consistency. This would break outside code using it. But it is an internal method and I cannot see why one would fiddle with it, so I guess it is safe.