- joined-table inheritance will now generate the primary key

- joined-table inheritance will now generate the primary key
columns of all inherited classes against the root table of the
join only.  This implies that each row in the root table is distinct
to a single instance.  If for some rare reason this is not desireable,
explicit primary_key settings on individual mappers will override it.

- When "polymorphic" flags are used with joined-table or single-table
inheritance, all identity keys are generated against the root class
of the inheritance hierarchy; this allows query.get() to work
polymorphically using the same caching semantics as a non-polymorphic get.
note that this currently does not work with concrete inheritance.