Switched Tilemap to use user-specified layer.

Pull request relating to issue #183.

Tilemap was using the current layer even if a layer was specified as a
parameter in getTile/setTile. Changed it to use the user layer if one is
specified.

 Not sure is this change screws up any of the Tilemap loading, however. I checked the tilemap examples and they all stilled seemed to work, but I'm not familiar with the Tilemap loading code in detail.