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.
This should be in the paragraph above as it doesn't stand alone. Plus it's probably not different to all other providers, just the ones we support. I thought about how this could be reworded, and came up with:
```
Access to Mapbox web services requires an access token and a
map ID. See https://www.mapbox.com/developers/api/ for details.
```