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

For customized initialization (on a per-instance basis) of backing view/layer. See #108.
What's the advantage of using `_viewClass` instead of `!_flags.isLayerBacked`?

When `initWithLoadBlock:` is used, we don't yet know if our node is layer-backed or not. But we do know that `_viewClass` is non-nil when it is safe to access `[ASDisplayNode view]`.