ActiveRecord-style model callbacks

We've been using [AR-style](http://edgeguides.rubyonrails.org/active_record_callbacks.html) `before`- and `after`-callbacks around here, and I thought I'd try to upstream it, if possible.

I've got a simple implementation of before/after Create, Update and Destroy. What do you think of the syntax? Am I on the right track for implementation?


__To Do:__

- test that before/after callbacks are in the right sequence with the operation, not just each other (like the lifecycle test)
- Other AR callbacks? before/afterValidation, before/afterInitialize, before/afterFind
- accept functions or strings as callbacks