Pulled main BossView functionality into a mixin.  This was inspired by c...

Added Marionette.CompositeView BossView.  Pulled out main BossView functionality into a mixin and use it in both the ItemView and CompositeView BossView definition. 

If this is something you are interested in, I can clean up test cases and right new ones for the Mixin/CompositeView

The mixins can be made very simple if a mixin library such as CocktailJS is introduced:
https://github.com/onsi/cocktail