Refactor Behavior to subclass Object

Cleanup Behavior so that it subclasses Object. This was by far the most red of the three refactors. When all is said and done, Behavior is one of the tiniest classes we [have](https://github.com/jasonLaster/backbone.marionette/blob/6e51fc787edc0907b19d01fe5e0190aaff1acbcf/src/behavior.js). Who knew?!?!


Heh, yeah, I know. I'm just glad we're getting rid of the IIFE.