added media qy for small devices (320px)

added media query to fix the bottom menu on small devices
before:
![screenshot-area-2014-07-15-220306](https://cloud.githubusercontent.com/assets/291082/3593399/f8f476e8-0c84-11e4-9654-73c925e42b1f.png)

after:
![screenshot-area-2014-07-15-220207](https://cloud.githubusercontent.com/assets/291082/3593397/db5514c6-0c84-11e4-9396-77c07a426fab.png)