JS Plugins and JSON Handlers

This add two new things to the IPython Notebook:

* JavaScript plugins: third party JS+CSS code that plug into the Notebook.
* JSON handlers: JS plugins can send JSON to the browser using `publish_json` and declare a JS function that will be the handler for that message.

See:

https://github.com/ipython/jsplugins

For details and an Online Python Tutor example.