def layer(name, srs = nil)
      layer = Mapnik::Layer.new(name, srs)
      layer.map = self
      yield layer
      layers << layer
    end
def layer(name, srs = nil)
      layer = Mapnik::Layer.new(name, srs)
      layer.map = self
      yield layer
      layers << layer
    end
Creates and yields a new layer object, then adds that layer to the map's
 collection of layers. If the srs is not provided in the initial call,
 it will need to be provided in the block.
 @return [Mapnik::Layer]