- most of the __init__ decoration has been removed from mapper, save for

- most of the __init__ decoration has been removed from mapper, save for
that the mappers all get compiled when an instance of a mapped class is first constructed.
the SessionContextExt extension gets all the "add object to the session" logic now and the
_sa_session and _sa_entity_name arguments only apply to when the SessionContextExt is in use.
Some extra methods to MapperExtension to support __init__ decoration.
- assignmapper loses "join_to", gains "join".  id like to replace all those methods with just "query"
but i think they are too popular, so it should probably get filter(), filter_by() also.