title: adding b2world to a CCLayer
body: <p>I have a <code>CCScene</code> that has many <code>CCLayers</code> on it .</p>
  
  <p>1.
  I would like to add a specific layer that has a <code>b2world</code> in it (<code>box2d</code>) .
  I need it to be a completely separate layer that i add above the current scene,than remove .
  Can i just create a world in a layer class and add that class as a layer ? or i need to have a scene?</p>
  
  <p>2.
  How you define the <code>gravity</code> vector to be 90 degrees to the iPhone screen ? (its usually parallel to the screen and point down to the home button,and not "3d" . ) .</p>
  
  <p>Thanks a lot .</p>