title: Swift physicsbody edge recognition issue
body: <p>The following code recognizes the <strong>bottom and top edge</strong> of the scene and the ball bounces off as expected. However, the <strong>left and right edges</strong> of the scene are breached all the time. The ball goes off screen and then eventually returns back if enough force is applied. It is as if the edges of the scene are beyond the edges of the iphone simulator window. Can someone help? Thanks.</p>
  
  <pre><code>import SpriteKit
  
  class GameScene: SKScene {
      override func didMoveToView(view: SKView){
          self.physicsBody = SKPhysicsBody(edgeLoopFromRect: self.frame)
          backgroundColor = UIColor.cyanColor()
          var ball = SKSpriteNode(imageNamed: "ball")
          ball.position = CGPoint(x: self.size.width/2, y: self.size.height/2)
          self.addChild(ball)
          ball.physicsBody = SKPhysicsBody(circleOfRadius: ball.frame.size.height/2)
          let push = CGVectorMake(10, 10)
          ball.physicsBody.applyImpulse(push)
  
      }
  }
  </code></pre>