Group symbolizer parse and render

This adds functionality for parsing and rendering group symbolizer (mapnik/mapnik#2160). The goal is to be able to create group symbolizers in Carto CSS. The basic attributes are covered in mapnik/mapnik-reference#61. The changes here are focused on the ability to match child rules to a group symbolizer and render them as xml content within the parent group symbolizer along with a group layout.

See https://github.com/mapnik/mapnik/wiki/GroupSymbolizer for more information on group symbolizer.