Website redesign

This new design cuts a lot of redundancy from the current site, and (IMO) looks much nicer.

A few points of note:

* The downloads page assumes that the binaries are hosted using GitHub releases (we can change this easily enough, but I think this is a good idea).
* Comments are provided via disqus, with whom I have created an OpenRA account.
* HiDPI (retina) images are used on browsers/devices that support them.
* It looks and functions fine with javascript and/or css is disabled.

A demo is available at http://pchote.github.io/OpenRAWeb/