Added StamenSource with watercolor and toner layers
Updates to StamenSource plugin
- Add terrain layer (note: US Only)
- Added new "flavors" of toner, terrain layers (eg: labels only)
- Add numZoomLevels, maxResolution as per discussion in Issue #139
- Verify some settings against sample provided by Stamen Design at
  http://maps.stamen.com/js/tile.stamen.js
Remove maxExtent, maxResolution, units from StamenSource configuration
Merge pull request #181 from benmccall/feature/stamensource

Added StamenSource with terrain, toner, and watercolor layers