Added Backbone.Super, custom templates

I've added the Backbone.Super lib, and rearranged all stuff to test the
inheritance. I've added to the most generic Views (WidgetItemView,
WidgetCompositeView) a common behaviour: custom templating. So the user
can now provide a custom template for the widget. Warning: if there is
something binded in the ModelBinder that it is not in the template, this
will throw an error, and the widget will not be build.