Make Facebook sharing prettier by using image.

This made me sad: 
![screenshot 2014-02-09 19 40 03](https://f.cloud.github.com/assets/129761/2122141/13824d28-9205-11e3-8dfb-7207331f0bd4.png)

So this should fix it, based on the [OGP specs](http://ogp.me/).  This might need to be an absolute URI, as opposed to the current relative one.