title: Centre a SKLabelNode on a SKSpriteNode
body: <p>I have an SKLabelNode that is the child of a SKSpriteNode because I'm trying to create a Button class to create buttons in an easier way. I've tried a couple of things using the anchor point of the SKSpriteNode, but I don't quite understand exactly what is going on. How do I centre the label onto the sprite (it's parent node)?</p>