add GitHub commit onebox

This will change this link: `https://github.com/discourse/discourse/commit/ee76f1926defa8309b3a7ea64a25707519529a13` into the following onebox:

![Screenshot_03_04_13_03_23](https://f.cloud.github.com/assets/362783/331803/fe49f764-9bfe-11e2-8bad-8ff15edc5630.png)

Feedbacks much appreciated :wink: