Natively process OSM data sources

[mattdelsordo]

At the time of writing, you can 100% render OSM data in Tilegarden, but you have to run it through osm2pgsql first so that Tilegarden can read it from Postgis. Mapnik does allow for using OSM data with a plugin, but it's not compiled in by default in the C++ version and it's unclear whether it exists in node-mapnik.

1. Plug Tilegarden into an OSM data source and see whether it renders that data or not.
2. Figure out how/if OsmPlugin can be compiled into node-mapnik when installing dependencies.