delete using sql when there are no destroy callbacks defined

@ColinDKelley @jpterry  what are your thoughts on this? It will make :dependent => destroy smarter on a has_many association by taking the sql route for deleting when the model in question has no destroy callbacks. So when we do something like ```BulkPendingEmail.last.destroy```,  and we have dependent => destroy defined it should delete the recipients this way:

DELETE FROM `bulk_pending_email_recipients` WHERE `bulk_pending_email_recipients`.`bulk_pending_email_id` = 21 AND `bulk_pending_email_recipients`.`id` IN (1, 2, 3, 4, 5, 6)