Proper build environment

[Tis the season for PRs](http://24pullrequests.com/), so I thought I'd check out MarkUs.

I added a Gulp-based build environment. This should make things easier to develop -- it looks like minification was done by hand before! -- and it also shaves of about a third of the page load size.

To use it:

 * Install Node.js if you haven't got it already.
 * Run `npm install -g gulp` to install the Gulp CLI tool (once per machine).
 * Run `npm install` once from the repo to install build dependencies in the repo.
 * Run `gulp` to build your assets, or `gulp dist` to compile and minify all assets (which takes a bit longer).