Refactor with HTML5 features and improved responsiveness

Saw an opportunity to contribute here when I read that there were no plans to support IE. I've made an effort to preserve the _intent_ of the design while improving on some shortcomings, mostly in regards to scaling on small devices. There's some `display: flex` bling here and there to keep the browser's sizing well-behaved.

There's also a lot of HTML that's been cut out and simplified. I'm no good with Javascript, so I had to figure out what needed to be left in place by trial and error. Might've missed something.

Here's a before-and-after comparison, taken from a 7" Android tablet.

![before-after](https://cloud.githubusercontent.com/assets/5333316/3776106/d6942c4c-194f-11e4-9a1c-4806be300ccc.jpg)

The difference is even more pronounced on mobile.

I can't figure out how this is meant to be deployed, so the changes are only in the `src/` directory, which I assume propagates somehow to the top-level site.