Added back-to-top-link and twitter-,facebook,mailto-link in navigation

I've added a back-to-top-link and a few social-buttons in the navigation.
The social-buttons are just links to the Twitter- or Facebookpage.

![auswahl_001](https://cloud.githubusercontent.com/assets/6506725/3948724/01a0f8ac-26ac-11e4-80d3-d7c5b22e14e4.png)

To use this, just edit the index.hbs and rename the links.