Add background-size contain

This makes the background fit on mobile devices

Before:
![image](https://cloud.githubusercontent.com/assets/167882/3837995/5258ee9e-1dfb-11e4-9ce7-fbb5ed2fdb06.png)

After:
![image](https://cloud.githubusercontent.com/assets/167882/3837993/4b2df63c-1dfb-11e4-9edb-6292b738e9b7.png)