Add ``alter_queryset`` hook to ``get_list``

Added a hook point to get_list called alter_queryset. This allows you to apply things like prefetch_related to a queryset after all filtering and pagination has been completed.

Main reason for adding this is so that I don't have to override ``get_list`` in order to accomplish this.

There are no tests because the default implementation is a no-op. And this patch does not alter the current behaviour of tastypie in any way.