Refactoring intercut to share code with new overlap feature.
Refactoring intercut to share code with new overlap feature.

This adds a new function, overlap, which will project an
attribute from the cutting layer down to the base layer for
any feature which overlaps in area by more than some
configurable amount (default 80%).

mapzen/TileStache,moskvax/TileStache,mapzen/TileStache,moskvax/TileStache,moskvax/TileStache,mapzen/TileStache