Add BehaviorClass property

There are times when you want to circumvent behaviorLookup, and provide the Behavior class directly. One example of this is when building bundles with requirejs. It's nice to be able to specify in the dependency that you want a view to have a behavior.

```js
define(
   ['Backbone', 'OverlayBehavior'], 
   function(Backbone, OverlayBehavior) {

   var StatusOverlay = Backbone.Marionette.ItemView.extend({
       behaviors: {
           Overlay: {
              BehaviorClass: OverlayBehavior
           }
        },

        // other stuff
    });
});