New site design

Here's an initial pass at the markup and styling for the design I proposed in https://github.com/neovim/neovim.github.io/issues/1. 

I opted for a mobile-first methodology here, so you'll see styles defined for mobile followed by media queries for larger displays. This initial pass incorporates most of the content supplied by @jdavis. I assume we'll have more than one FAQ, so we need to flesh that section out, but I think we should be pretty close overall. Since we just have a single page for now, I opted to use the top nav space for links to the various external sites (GitHub repo, Google Group, and Twitter).

Preview available at: http://jasonlong.github.io/neovim.github.io/