Fix passing custom querysets to related managers.

* The custom manager was not assigned to _default_manager;
  get_first_user_defined_manager() always returned None
* The PolymorphicManager couldn't remember it's custom queryset;
  a RelatedManager creates a new instance of a manager, so the queryset
  parameter should be known at class-level, not object level.
* The old method of providing a custom queryset class has been deprecated.
Merge pull request #33 from vdboor/fix_related_queryset

Fix related queryset (please merge before #32)