Translate BAG 'kantoorfunctie' attribute to OSM tag building=office.

Previously 'kantoorfunctie' was translated to building=commercial which was
deemed inappropriate in too many occasions.

See for reference the discussion on the OSM forum:
[BAG Gemeentehuis e.d. als building=commercial?](http://forum.openstreetmap.org/viewtopic.php?id=25249)