Add onebox for GitHub pull requests

I'm tired of seeing my pr links on meta without the fancy box :frowning: 

This adds a onebox for GitHub pull request links.

It turns this:

`https://github.com/discourse/discourse/pull/988`

Into this:

![pr-onebox](https://f.cloud.github.com/assets/65323/627879/875b1c90-d061-11e2-8642-ddcaeb86282f.png)