Usage: add_timed_property(mixed property,int value,int duration)
i.e. add_timed_property("property",1,1000);
This adds a property which will disappear after 1000 hb's on a player
or NPC or a roughly comparable time on rooms or items.
Note that timed properties survive logouts so
are quite useful for regulating things like quests, for example
you can prevent a quest being re-done until the players
has put in two more days.