Asset cache busting

This patch sets the `Cache-Control` header and adds cache busting URLs for assets served by comfy. Compatibility with old URLs is preserved so that people who may have hard coded URLs are not disadvantaged.

The new URLs in a link element look like this:

``` html
<link href='/cms-css/1/campaign/1399337593.css' media='screen' rel='stylesheet' type='text/css' />
```

The old URLs continue to work:

``` html
<link href='/cms-css/1/campaign.css' media='screen' rel='stylesheet' type='text/css' />
```

The timestamp is derived from the updated_at on the layout. This opens up the possibility of seeding these assets through something like Cloudfront in the future.