Add Backbone.BabySitter

Now that we are to be splitting up `DashboardController` into a model and view (#77), and rendering the dashboard and its widgets on the client side (#67), we will need a way of managing the widget view nested in a dashboard view. To avoid re-inventing the wheel, we can use [Backbone.Babysitter](https://github.com/marionettejs/backbone.babysitter) for this.