Fix asset compilation for images

Currently, our CSS file contains the following:

```css
a[rel="external"]:after {
  background-image: image-url(external-link.png);
  background-repeat: no-repeat;
  content: "\A0\A0\A0\A0\A0";
  background-position: right 3px;
}
```

Obviously this isn't valid CSS, and it's happening because Sass doesn't know to compile an `image-url`. To fix it, we include the Compass gem and set up Compass correctly so that it knows where it should be looking for images. It then provides proper cachebusting of those `background-image` attributes, which is nice.

You can verify this pull request with the following:

```bash
cd /var/apps/spotlight
# on master
grunt sass:development
grep image-url public/stylesheets/spotlight.css # happens 4 times
# checkout this branch
grunt sass:development
grep image-url public/stylesheets/spotlight.css # no longer there
grep url public/stylesheets/spotlight.css # lots of happy images with query params
```