Named Layers update

Layers are now referenced by name from within scenes.  New layers must
be passed a name string.  Layers within scenes can be accessed at
layers[layerName].

Existing layers can added to scenes using the scene's
.addLayerObject(layer) method.

The scene's .layerOrder property is an array of strings which specifies
the order in which layers should be updated and rendered.

New scenes contain one layer, called "background", and with a
.layerOrder array ["background"].