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}`.

Some functions don't have docblocks because they're functions available in the code but not described in the API, and since Mapbox's code doesn't bare comments, I've just added the method definitions alone.

This is my first contribution here so, comments encouraged.