title: how to listen to array submodel's value change
body: <p>I have a model define like below.</p>
  
  <pre><code>var A = Backbone.Model.extends({
      initialize: function(){
         this.on('change:users.name',this.onUserNameChanged)
      },
      onUserNameChanged: function(){ alert('name changed')
  });
  var a = new A ({
     id:1,
     name:'test',
     users:[
     {
        id:2,
        name:'u1'
     },
     {
        id:3,
        name:'u4'
     }
    ]
    })
  </code></pre>
  
  <p>I want add event on the each user name change in Model define.
  I have no idea to do this.It's seems hard to me.</p>