Implement exit scripts, room enter/leave scripts

Currently for doing things like:
executing a command on entering/leaving a room, executing a command on entering/leaving an area, mapping special exits to cardinal directions (e.g. 'climb stairs' for up), pulling a level before going a direction, etc.

The user must put together their own lua handling routine making use of RoomUserData. I propose we take some common use scenarios and incorporate these into the mapper itself and extend the exits/room info GUI to make them accessible to the user.