Chroot jail: creation code, back-end and ajax hook.

# The Jail Update

- `Makefile`: sequence of commands to create a debian chroot jail and populate it.
- `lib/sandbox-shell.js`: communicate with the jail and run arbitrary bash commands.
- `lib/fsapi.js`: addition of the ajax hook to run arbitrary commands.

Note that this change is Linux-only, but it should degrade gracefully when we are not on Linux, or even when there isn't a sandbox at all.

/cc @jankeromnes for review!
FIXME: write in background as often/soon as possible? It doesn't cost much, and writing often makes for better crash recoveries.