Add GitHub Ribbon and "Join Us"

This cover #448 adding a link to the mail list at the navigation bar.

I'm not sure if GitHub ribbon should be in our main site and still need to add the ribbon to V4 lessons.

![swc](https://cloud.githubusercontent.com/assets/1506457/2742485/6d7474da-c704-11e3-9bf8-2e12e7d8f7af.png)