Added Stamen map types, better Readme

Stamen's Toner, Terrain (in US), and Watercolor maps are now supported out of the box.

Also fixed the Bing Street map label (was missing Bing).

Also added folder of "Goodies" for GDAL powered TMS service (add in QGIS by doing a raster layer add, selecting one of these XMLs). They reproject, while the main bit in this plugin just does web Mercator projection.