Corrected behaviour to get listing from nested resource.

DETAILS:
1. Corrected behaviour to get listing from nested resource. Now
/parent_resource/<pk>/nested_resource/ will now work :).
2. Changed parent_*obj_get function to follow the current Tastypie's
parameter norms of using bundle instead of request.
3. Minor code realignment.
Removed nested authorization.

DETAILS:
With the change of authorization mechanism in Tastypie 0.9.12 that
allows finer grain authorization control, the current implementation of
checking authorization over nested resources becomes out-of-sync with
the new design.

Ultimately I think that it should be removed completely - as I don't
believe that the nested resource should have any difference in handling
authorization behaviour from the original resource.