Exceptions from the (receiver, exception) tuples returned by
``send_robust()`` now have always their traceback attached as their
``__traceback__`` argument.
Make use of `weakref.finalize` and `weakref.WeakMethod` on python 3.4.
Simplified the removal of receivers, the old function looked overly
complicated.
Many thanks go to Antoine Pitrou for helping me to debug and explain all
the failures I ran into while writing that patch.
Added comments in the three empty models.py files that are still needed.
Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.
