- fixed bug whereby session.expire() attributes were not

- fixed bug whereby session.expire() attributes were not
loading on an polymorphically-mapped instance mapped
by a select_table mapper.

- added query.with_polymorphic() - specifies a list
of classes which descend from the base class, which will
be added to the FROM clause of the query.  Allows subclasses
to be used within filter() criterion as well as eagerly loads
the attributes of those subclasses.

- deprecated Query methods apply_sum(), apply_max(), apply_min(),
apply_avg().  Better methodologies are coming....