Save center UI

![savecenter](https://cloud.githubusercontent.com/assets/83384/4273854/9c1e2078-3cee-11e4-8a85-5565b4d23a0e.png)

Addresses https://github.com/mapbox/mapbox-studio/issues/279, https://github.com/mapbox/mapbox-studio/issues/836.

- [x] tests