Added backgroundTile attribute to WorldCanvas.
Added background-image property to level loading code.  Put
background-image in the level.cfg file.
Added DARKENING_VALUE constant.
Added deepCopy(BufferedImage) method.
Added backgroundTile darkening support.
Adjusted DARKENING_VALUE in GameConstants.
Added null checks of background Tile.
Created Physics class.
Added CollisionGroup attribute to WorldCanvas.
Added setUpCollisionGroup method.
Overrode the Canvas.add(e).  e is now added to the collisionGroup.
Used super.xxx instead of copied code in some methods.
Added Physics constants.
Added PLayer attribute to WorldCanvas.
Added getter and setter for collisionGroup.
Added incomplete gravity support.
Updated Puzz Blocks Dev Log.