- propagate flag on event.listen() results in the listener being placed

- propagate flag on event.listen() results in the listener being placed
in a separate collection.  this collection also propagates during update()
- ClassManager now handles bases, subclasses collections.
- ClassManager looks at __bases__ instead of __mro__ for superclasses.
It's assumed ClassManagers are in an unbroken chain upwards through __mro__.
- trying to get a clear() that makes sense on cls.dispatch
- implemented propagate for attribute events, plus permutation-based test
- implemented propagate for mapper / instance events with rudimentary test
- some pool events tests are failing for some reason