Major refactoring of saving, notification. * Refactored the save widget so that the notebook doesn't depend on it. Now the notebook emits events and the save widget observes those events. * Created a new event system for all IPython events (events.js). We should start to use this to allow our classes to be loosely coupled. * Created a new notification widget that should be used for all notifications. Uses new event system. * Removed the kernel status widget. * All kernel status message use new event/notification system. * The print notebook view works again.