Mapbox definitions

This is a first stab at the Mapbox library definitions. I tried adding tests but it seems inconsistencies in the leaflet definitions (which Mapbox extends) prevent even the official examples from working. I'll need to send a separate PR for that – basically the Leaflet definitions enforce instances of `LatLng` where Leaflet actually accepts per example `[10, 20]` or `{lat: 10, lng: 20}`.

This is my first contribution here so, comments encouraged.