Render natural=rock areas in light grey

Render natural=rock areas in light grey.

Example location: http://www.openstreetmap.org/?lat=53.1577&lon=-1.66033&zoom=18

(Note that this isn't present in the old osm-mapnik style either. If you want to keep the master branch purely compatible, an option could be merging this into a branch for "post-compatible" map features.)