Prettify the navbar when browsing on mobile devices.

When accessing exercism.io from a mobile device, the navbar wraps around the screen and doesn't look nice. Make it look better by using bootstrap's collapse plugin and collapsing the navbar when on mobile devices.

Here's a before screenshot using Chrome's mobile emulation:
![screen shot 2014-07-15 at 9 15 42 pm](https://cloud.githubusercontent.com/assets/77174/3593959/a91b9a3c-0c90-11e4-8bd2-4f6a13ab12cc.png)


Here's are two after screenshots (again using Chrome's mobile emulation)

Collapsed:
![screen shot 2014-07-15 at 9 16 52 pm](https://cloud.githubusercontent.com/assets/77174/3593968/d5df5dce-0c90-11e4-826a-2f0c32ae5cac.png)

Uncollapsed:
![screen shot 2014-07-15 at 9 17 17 pm](https://cloud.githubusercontent.com/assets/77174/3593970/d98ebd98-0c90-11e4-8cbd-2a137d8205aa.png)


Thanks! :heart: