Add JavaScript language support (via node.js)

This change adds JavaScript language support. By default, it's disabled for submissions.

I've tested both in a chrooted environment, and with chroot disabled.

My Node.js version is 0.10.25. To install Node you can [follow these instructions](https://github.com/joyent/node/wiki/Installing-Node.js-via-package-manager). I couldn't get it to install from a package in the "extended" chroot provided by `dj_make_chroot`, so I installed from source in the chroot jail.