Fix typo in leaflet tile layer creation

The min zoom level wasn’t set to the right value
Remove dead code and fix typos in spec case
Substitutes GMaps basemaps w/ Leaflet layers

The substitutes are equivalent basemaps using the Leaflet provider
instead.
Replace substitutes with new stamen maps

Also DRY’ed the code a little bit while at it
Update NEWS with what changed
Merge branch 'develop' into 188-gmaps_basemaps_substitutes
Merge pull request #189 from CartoDB/188-gmaps_basemaps_substitutes

188 gmaps basemaps substitutes