- Fixed an 0.9 regression where ORM instance or mapper events applied

- Fixed an 0.9 regression where ORM instance or mapper events applied
to a base class such as a declarative base with the propagate=True
flag would fail to apply to existing mapped classes which also
used inheritance due to an assertion.  Addtionally, repaired an
attribute error which could occur during removal of such an event,
depending on how it was first assigned. [ticket:2949]