Enable the IPython Notebook to display messages from other clients.

This feature was discussed in #6123, but it doesn't look like anything was ever incorporated into the IPython Notebook.

Here's a brief overview of the changes:

- Display of messages from other clients can be toggled on and off from within a notebook, either using the ``<M-m>e`` keyboard shortcut in the web UI, or through the option in the ``Kernel`` drop-down menu.
- ``notebook.js`` controls whether messages are displayed through a callback that is invoked from ``kernel.js`` when no callbacks are available for a message.
- The UI displays ``execute_input`` messages originating from an other clients in new cells at the end of the notebook. Output messages (``execute_result`` et al.) will only be displayed if a cell exists with a matching message ID.

Pending design questions:

- Should each ``execute_input`` message cause a new cell to be created?
- Should new cells be placed at the end of the notebook, or elsewhere? If the latter, what criteria should be followed?