Use new Map object to avoid duplicate layer names.
Use new Map object to avoid duplicate layer names.

tomhughes/node-mapnik,mapnik/node-mapnik,mapnik/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,tomhughes/node-mapnik