Implement RethinkDB Changefeeds

[Morgul]

The goal is to use RethinkDB's changefeed support as our synchronization mechanism.

As far as I can see it, this involves removal of the update function, instead listening for the change event from thinky.

@whitelynx: Looking at the code for BaseEntity, I think we just implemented our behavior system a little poorly. We should be binding to properties on entityDef directly, not overwriting it's save function, or any of that BS. We should probably audit the behaviors, and make sure that we're not doing too much work that we shouldn't need to be done.