New callbacks

[John D. Hume]

Matt Courtney (the Conjure guy, http://github.com/macourtney/Conjure)
has added support to clj-record for a whole bunch of callbacks. Most
of the details and some discussion are here
http://github.com/macourtney/clj-record/commit/0f4beabeabd4cd88f26f7d1c217e59128126f6ec
and in the commits following.

My only concern was that destroy-records went from being a single
delete query to a select, an iteration, a delete, and another
iteration, so, in alignment with ActiveRecord behavior, I've
introduced delete-records, which doesn't run the destroy callbacks.

Let me know if you have any questions or experience issues with these changes.
-hume.

--
http://elhumidor.blogspot.com/