session now propigates to the unitofwork UOWTransaction object, as well as mapper's save_obj/delete_obj via the UOWTransaction it receives. UOWTransaction explicitly calls the Session for the engine corresponding to each Mapper in the flush operation, although the Session does not yet affect the choice of engines used, and mapper save/delete is still using the Table's implicit SQLEngine.
changed internal unitofwork commit() method to be called flush().
removed all references to 'engine' from mapper module, including adding insert/update specific SQLEngine methods such as last_inserted_ids, last_inserted_params, etc. to the returned ResultProxy so that Mapper need not know which SQLEngine was used for the execute.
changes to unit tests, SelectResults to support the new Query object.