Thing Rendering and Example

Added new Things:
	Stairs - Allows Agents stepping on them to increase or decrease their
z-coordinate depending on direction stepped and direction of the Stairs

Added rendering for Things, of which Stairs are an example. 
Still coming is an adjustment to the Step action that makes stairs have
an actual effect on movement (right now they are simply a rendered image
of stairs that any Agent can step on as if they were a regular terrain
tile)