Added more comments to widget model JS
Added on_view_displayed and on_close callbacks to widget manager.
Properly dispose of widget model.  Delete comm's ref to
the widget model so the GC will collect it when it is
no longer reference elsewhere.
Added support for custom widget msgs
Fixed typo in widget model code causing notebook to not load
Cleaned up _make_callbacks method of widget model class
Remove some empty space
Added TextBox submit event
Fixed a bug that didn't allow callbacks to set a property
on the widget that triggered the callback.
Add scroll_to_bottom method for TextAreaView (StringWidget).
Added an example of how to create non-blocking widget code
Make sure backbone events fire.
If $el is set to something else, make sure all the backbone machinery
knows about the new DOM element.
Fixed typo in container construction.
Use setElement to set the view's element properly.
Make sure DOM element ids start with alphabetic characters, not numeric.
Changed add_class and remove_class to use messages instead of stateful communication
Changed button to use custom messages instead of state to communicate events.
Updated events example to reflect changes to button.
Changed on_submit to use custom messages instead of stateful communcation
Made scroll to bottom use msgs
allow multiple msg handlers
added send to view to automatically hookup callbacks
changed send signature to optionally accept a cell to connect callbacks