switch icons

![screen shot 2014-10-02 at 3 16 17 pm](https://cloud.githubusercontent.com/assets/108094/4496801/5d2fa606-4a69-11e4-859c-9120127ff4a6.png)

switches icons, see https://github.com/mapbox/mapbox-studio/issues/540#issuecomment-52988541