Update stamen maps license per https://github.com/Citytracking/maps.stamen.com/commit/4b661b285f6c1fcc513afdb720c74a4b87b93003

The nice folks at Stamen clarified that Stamen Maps is BSD licensed.
Merge pull request #34 from obra/patch-3

Update stamen maps license per https://github.com/Citytracking/maps.stam...