WIP: %kernels magic

@fperez requested this, and here's a functional version of it. 

This PR adds a `%kernels` magic, which returns a list of notebooks which have an active kernel, and adds a shutdown button for it, just like on the dashboard's Notebooks tab. The difference is that this will show *all* notebooks with an active kernel, not just the ones in the current subdirectory, and it will list the entire (served) path for the notebook.

Features:
- [x] list of active notebooks updated when shutting down a kernel.
- [x] above actually updates all results of %kernels magic for this notebook
- [x] link to the current notebook is inactive
- [x] allow manual update (in case

TODO:
- [ ] sane representation in console
- [ ] `%kernels kill <id>` (not yet implemented)

optional:
- [ ] refactor NotebookList so we can add an Active Kernels dashboard tab (depending on feedback)
- [ ] window blur and focus autorefresh (like the dashboard view)


(I'll upload a screenshot shortly)