title: Make rotation stop realisticly in Swift
body: <p>I have a <code>square = SKSpriteNode()</code> that turns that rotates 360° when you touch it. I want to stop the spinning when you touch it again. 
  Now I would make <code>square</code> rotate through the <code>SKAction.rotate</code> but how can I stop the spinning realisticly meaning that I want the sprite to spin slower and slower until it stands still.</p>