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.
I think we handle this in the `Navigator`