Multidirectory support for the Notebook

This PR is a continuation of PR #3619 from @Zsailer. We are moving it over to an IPython hosted branch to allow everyone on the dev team to help with the work.  Also of reference is the [IPEP 16](https://github.com/ipython/ipython/wiki/IPEP-16%3A-Notebook-multi-directory-dashboard-and-URL-mapping) on this topic.