Share icons: coloured & show at top of articles

Step 1 of the share icon changes.

- Share icons are now coloured
- Share icons now show at the top of articles on mobile/tablet 

![screen shot 2014-10-23 at 11 29 13](https://cloud.githubusercontent.com/assets/2236852/4751563/7a336b70-5a9f-11e4-9214-16856aadc0c6.png)

// cc @mattosborn 
 
I would suggest abstracting these colours out into variables, so they can be used across the code bas.e