Fix #356 - Added mapbox tiles and set as default.
Fix #356 - Improved mapbox tiles, adding a 256x256 server that would be default for lower than hdpi and the 512x512 for the other devices. New options are "mapbox" and "mapbox retina".
I have tested 256x256 tiles on a hdpi to see if would be good to include this res in the low group but there is a very important difference, specially when zooming.
Merge pull request #380 from vreixo/issue356

Fix #356 - Added mapbox faster servers with 256x256 and 512x512 tiles.