title: checking if an SKNode is running a SKAction
body: <p>How can I check whether a <code>SKNode</code> is already running an action before running an action on it?
  I want to be able to do something like...</p>
  
  <pre><code>if (![mySprite isRunningActions]) {
      [mySprite runAction:action]; 
  }
  </code></pre>
  
  <p>If there is no built in way I'm thinking of creating a new <code>BOOL</code> property for holding the action state.</p>