add parking_entrance rendering

I was looking at KeepRight and came across this error and the one next to it:

http://keepright.at/report_map.php?schema=86&error=48351546

These nodes are the entrance and exit to an underground car park. I tried to fix this by changing the nodes from amenity=parking to amenity=parking_entrance (http://wiki.openstreetmap.org/wiki/Tag:amenity%3Dparking_entrance) and adding entrance tags to both. I've also suggested to KeepRight that oneway ways which end/start with nodes tagged with entrance=* be not flagged as errors.

Because I've changed the OSM data, it would be good if these nodes were still rendered as parking, so this pull request adds a new symbol (as suggested on the above OSM wiki link) and instructions to render this on the 'main' entrance. Of course, this could be extended to include private parking entrances in a different colour, etc. The pull request currently has no icon for exits - having separate entrance/exit/both(?) signs is a bit more complicated, especially when you come to render the name tag, so more complex tagging might be needed. (name for entrance=main is rendered, but this doesn't necessarily mean the entrance is not an exit ...)