Integrate the Hem Assets toolchain into the IPython notebook

This pull request adds an assets toolchain to the notebook.  This makes it easy to develop frontend features while also allowing for fast response times while deployed.

The way this code works, there should be no change to the existing workflow for developers.  When IPython is deployed, [https://github.com/spine/hem] can be invoked via `hem build` to bundle all of the javascript files into a single file named ipynb_application.js.  The ipython notebook is then started with the ``--compress-assets` option enabled.

`--assets-domain`  another command line option was added to enable serving of the js from the notebook from a separate url.  This makes it easy to put ipython notebook assets on a CDN.

I would like feedback on this feature.  We implemented this at Continuum Analytics on wakari to greatly speed up the loading of our notebooks.