Added "loadNode" block property to ASDisplayNode for customized initiali...

For customized initialization (on a per-instance basis) of backing view/layer. See #108.
Since `_layerClass` is no longer set inside of `-(void)_loadViewOrLayerIsLayerBacked:(BOOL)` (except when `initWithLoadBlock:` is used of course), we must take care to set `_layerClass` when `-(void)setLayerBacked:(BOOL)` is called.