Add Mapbox tiles retrieval

Adds a class for retrieving mapbox tiles. 

This class looks a little different to the other tile classes that inherit from `GoogleTiles` because Maptiles' approach to map tile provision is user oriented, so a given user's API access token and an ID for a map that they have created needs to be supplied.
Sorry - should have said before(!), but X, Y, and Z really should be different so we can distinguish them in the final URL.