Behaviors support

Opening this PR for commentary.

Event triggering should be moved to a behavior whenever possible. Views are still be able to override the event name to be triggered, although I'm beginning to wonder if that is really necessary: I'd rather have views concern themselves with ~~presentation~~ configuration only.

Marionette allows behaviors themselves to have behaviors, so keep that in mind if things start getting out of hand, we can always break up complex interactions using composition.
Behaviors work as event proxies, so both callbacks (view + behavior) will be called on the trigger.