Fix mobile hamburger button

Will fix #11.

Old behaviour: 
![old](https://cloud.githubusercontent.com/assets/7784737/5519777/c8249452-8917-11e4-9d68-80a4398fd4a1.gif)

New behaviour:
![new](https://cloud.githubusercontent.com/assets/7784737/5519778/cb0131bc-8917-11e4-9307-62b70e9182b4.gif)

Something something cropping