Refactor to make sure get_subclass() is on QuerySet

There's an edge case where you might want to call get_subclass() on a
QuerySet the same way you can call get() after you have already called
various filter/exclude methods.
Add test to verify get_subclass() on QuerySet
Merge pull request #82 from tswicegood/fix-get_subclass

Refactor to make sure get_subclass() is on QuerySet