Flushed out the Person class more. Added a global RNG for convenience.
Slight change to nextI(min, max) to avoid bias towards smaller numbers.
Added isWalkable function to support crosswalks, which are sometimes
illegal to cross. I'll probably also make it so that crosswalks have a
timer or some way to stop people from walking on the walks.
Crosswalk stuff added! TODO: make people follow an intelligent path.
Their current path is pretty dumb looking.