Add OSM GPS Tile Layer as a background option

The GPS Tile Layer ([which looks like this](http://bl.ocks.org/ericfischer/raw/713d24985c9a4a085629/#16/52.5104/-1.8617)) produced by MapBox is REALLY useful. Not only does it give a really solid "ground truth" for tracing roads and easy alignment of other imagery, but it helps understand directions and flows too. It would be great as a standard option in Potlatch2...