Responsive left sidebar

![screen shot 2014-12-24 at 10 44 37](https://cloud.githubusercontent.com/assets/92554/5546684/edc05112-8b59-11e4-8091-a341e337a107.png)
![screen shot 2014-12-24 at 10 44 42](https://cloud.githubusercontent.com/assets/92554/5546682/edbe419c-8b59-11e4-9e00-833e1edcbaa8.png)
![screen shot 2014-12-24 at 10 44 56](https://cloud.githubusercontent.com/assets/92554/5546683/edbff97e-8b59-11e4-9f69-5b15c0c1db97.png)