def style(name)
      style = Mapnik::Style.new
      yield style
      styles[name] = style
    end
def style(name)
      style = Mapnik::Style.new
      yield style
      styles[name] = style
    end
Creates and yeilds a new style object, then adds that style to the
 map's collection of styles, under the name passed in. Makes no effort
 to de-dupe style name collisions.
 @return [Mapnik::Style]