Managing assets like a boss

Really excited for the new CfA website to be using a static-site generator because it will make updating the site much, much easier. In that spirit, this PR uses Jekyll's assets (get it?!) for managing the asset compilation process:

- that pesky makefile and its hidden dependencies (jsbin, sass) are a thing of the past
- no longer commit generated css/javascript to the repo
- assets get compiled/minified during the normal build process
- `jekyll server --watch` will automatically update assets during development
- the Gemfile is perfect for documenting dependencies and locking their versions
- prepares you for continuous deployment nirvana

I did have to move all the assets in to the `_assets` directory. Also, the assets get a md5 hash appended to them during the build process, which is great for breaking caches on update... but also necessitates using [jeckyll-assets](https://github.com/ixti/jekyll-assets#how-to-use-jekyll-assets]'s helpers and the `asset_path('image.png')` mixin in your sass. Though this is a small price to pay for all the benefits above.