Add option to sort into inserts/updates to bulk_save_objects

Add option to sort into inserts/updates to bulk_save_objects

Added new flag :paramref:`.Session.bulk_save_objects.preserve_order` to the
:meth:`.Session.bulk_save_objects` method, which defaults to True. When set
to False, the given mappings will be grouped into inserts and updates per
each object type, to allow for greater opportunities to batch common
operations together.  Pull request courtesy Alessandro Cucci.

Change-Id: I0d041f7696cf733655a74beeceee3fa80640efd7
Pull-request: https://bitbucket.org/zzzeek/sqlalchemy/pull-requests/6