Basic floor traversal added

Floor Object now has a class variable storing floor depth.
Stairs up only created on floors with depths >= 1.
Added method to Game to traverse floor when Stairs are encountered.

Still need to connect stairs spatially (same x/y coordinates), and stack
them in pairs.
Need to check for collisions upon hero arriving at new floor.
Need to refine conditions for contact with Stair Objects.
Added to the TODO list

Level traversal functionality