initial implementation of multipolygons. most relevant code stolen from iD project (thx guys!)
robustified code against only partially loaded multipolygons.
more fixes for MP rendering
change fillColor too when highlighting features
added more natural=* values to trigger polygon display
extended tag-list for line<->polygon detection.
tags found using the "taginfo-wiki" database of Jochen Topf <jochen at remote.org>; thx!
support type=boundary relations (alias for multipolygon). show relations of relations properly
let this be for now
removed deprecated todo comments