Group symbolizer reference

This adds reference information for group symbolizer (mapnik/mapnik#2160). The goal is to be able to create group symbolizers in Carto CSS. This effort also requires a corresponding update to carto (mapbox/carto#349).

See https://github.com/mapnik/mapnik/wiki/GroupSymbolizer for more information on group symbolizer.
I am not sure that the above two attributes belong here in mapnik reference, because they do not correspond directly with options used by mapnik. The 'group-name' and 'group-class' css options are really aimed at the carto compiler, which will use the name and class just like a layer to build rules and symbolizers that go inside of the group symbolizer. I included them here mostly for simpler validation in carto.