Added Mizurski/ OSM Terrain for the US

Since I work for the USGS, I thought it would be cool to add the US Terrain tile map service that uses the USGS National Elevation Database to create this cool terrain tile map service. 

Example:
http://nbviewer.ipython.org/github/rsignell-usgs/notebook/blob/master/Cartopy/Cartopy_terrain.ipynb

I'm not sure about the best name for this new class.  I called it "StamenTerrain" instead of "Terrain" because stamen is providing this terrain service and there could be others.  But I yield to your judgement here. 

BTW,  Maybe since there are a lot of these services that follow the same pattern, would it be useful to have a generic class for these services that a user could instantiate with a base url?