Make IPython notebook display messages from other clients.

- Display can be toggled on and off using <M-m>e in the web UI, or through
  the "Kernel" menu.
- notebook.js controls whether messages are displayed through a callback
  that is invoked from kernel.js when no callbacks are associated with
  a particular message.
- The notification area is used to indicate whether messages from other
  clients will be displayed or not.