Add social promote links

@jospoortvliet @jancborchardt @karlitschek 

Will be available at /contribute#promote and we can add a redirect from /promote.

![screen shot 2014-07-02 at 15 12 10](https://cloud.githubusercontent.com/assets/660805/3456891/2bceaab2-01f3-11e4-9712-6adbe728f2ff.png)