Fix asset problem causing background-images to be broken

If the string passed to `asset-url` begins w/ `/` or `http` the digest is not added to the url (makes sense as you wouldn't want to fingerprint assets from a CDN).

However; since our assets can be accessed from various different hosts and through different reverse proxies we want to both begin our asset URL with `//` as well as add the digest.

### Example xp720tg7576

#### Before
![xp720tg7576-su-before](https://cloud.githubusercontent.com/assets/96776/5288267/62abac06-7ae8-11e4-96c1-3fa5eca1f474.png)
![xp720tg7576-license-before](https://cloud.githubusercontent.com/assets/96776/5288268/62ad50e2-7ae8-11e4-99b1-e9144888897f.png)


#### After
![xp720tg7576-su-after](https://cloud.githubusercontent.com/assets/96776/5288266/5f6d9608-7ae8-11e4-8f08-4f08dcfcdad0.png)
![xp720tg7576-license-after](https://cloud.githubusercontent.com/assets/96776/5288265/5f6c27a0-7ae8-11e4-8ceb-f0ff364d010c.png)