Add failing tests for get_objects_for_user with non-integer PKs

- Illustrate the scenario where the queryset will have non-integer PKs,
  and touch the points of failure in the code
Fix get_objects_for_user to work with objects having non-integer PKs
Change to Django-style doc-comments for non-int PK code
Add Steven DeMartini to AUTHORS
Merge pull request #253 from sjdemartini/fix/get-objects-for-user-non-integer-pk

Fix get_objects_for_user to work with objects with non-integer primary keys