Boat_sharing added

Adds an icon for boat-sharing POI. This is analogous to the well
establised car_sharing POI. It's accepted proposal:
http://wiki.openstreetmap.org/wiki/Tag:amenity%3Dboat_sharing
Even it might be a special interest tag, it does not harm even on a
general purpuse map, since it's alwas located in otherwise empty port
locations.