- added an assertion within the "cascade" step of ORM relationships to check

- added an assertion within the "cascade" step of ORM relationships to check
that the class of object attached to a parent object is appropriate
(i.e. if A.items stores B objects, raise an error if a C is appended to A.items)
- new extension sqlalchemy.ext.associationproxy, provides transparent "association object"
mappings.  new example examples/association/proxied_association.py illustrates.
- some example cleanup