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.
Then perhaps a squash would be nice to cover up my piecemeal reviewing. :wink: