Add Esri WorldImagery tile layer, make GMaps opt-in

Esri WorldImagery is as nice as Google Maps, but without the need of loading the Google Maps API:

![esri screenshot](https://cloud.githubusercontent.com/assets/344777/4211526/799f28a2-388a-11e4-9f74-bb2a47991047.png)

Google Maps is still loadable, but hidden behind a preference by default.