Add SSKTileableNode

This SKNode subclass renders its texture tiled,
meaning that it will replicate it to fill its size.
Add SSKStretchableNode

This node cuts its texture into 9 parts, and then
uses SSKTileableNode instances to fill its size
according to user-specified edge insets.

This works the same way as UIImage's
resizableImageWithCapInsets:, and allows for
dynamic resizing of textures without quality loss