title: how to watch for sub-child added event in firebase
body: <p>is there a way to watch for grandchild_added event with firebase?</p>
  
  <p>I've set up a observer for child_added using the node.js javascript library but it won't trigger for when a child of a child is added.</p>
  
  <p>I'm setting member presence info and would like to know when users enter a room.</p>
  
  <p>i.e. </p>
  
  <p>members/room/uname</p>