widen and pad share buttons on mobile

Re style the share share buttons for mobile - make them wider. But only on mobile. 

Before:
![desktop-before](https://cloud.githubusercontent.com/assets/986155/2775603/706d628e-cac7-11e3-9f69-8f25c36326c0.png)
![mobile-before](https://cloud.githubusercontent.com/assets/986155/2775607/79138710-cac7-11e3-9e64-d01587bc30d4.png)

After: