Option to configure MapboxAPITile

Allow MapboxAPITile to be configured for integration with Atlas Server. Sets API endpoint to tm.db for persistence.

To Do:

* [x] UI adjustments when offline
* [x] name: "Offline user"
* [x] MapboxAPITile checks at /status https://github.com/mapbox/mapbox-studio-internal/issues/170 & https://github.com/mapbox/api-maps/issues/525

![offline1](https://cloud.githubusercontent.com/assets/83384/5016386/e1842ff6-6a73-11e4-839a-9ae74549ad74.png)

![offline2](https://cloud.githubusercontent.com/assets/83384/5016387/e1870046-6a73-11e4-8418-3a940ba90c35.png)