Add style-level opacity - https://github.com/mapnik/mapnik/issues/314
Add style-level opacity - https://github.com/mapnik/mapnik/issues/314

CartoDB/mapnik-reference,florianf/mapnik-reference,CartoDB/mapnik-reference,gravitystorm/mapnik-reference,mapycz/mapnik-reference,mapnik/mapnik-reference,mapnik/mapnik-reference,mapycz/mapnik-reference,mojodna/mapnik-reference,mapnik/mapnik-reference,gravitystorm/mapnik-reference,florianf/mapnik-reference,mojodna/mapnik-reference