Add site scaffolding

Create site scaffolding with [Bourbon](http://bourbon.io/), [Neat](http://neat.bourbon.io/), [Bitters](http://bitters.bourbon.io/) and [D3](http://d3js.org). All of the static files and the (not-at-all-fleshed-out) base template are hosted in a new Django app, `hourglass_site` (using the same convention as [mirage](https://github.com/18F/mirage)).