title: Backbone Marionette Event Aggregator : how to listen at the App level to ItemView Event
body: <p>I'm new to Marionette and can't get my head around events...</p>
  
  <p>I have an ItemView which trigger an event, and I would like to receive this event at the application level, but when my app listen for this event, nothing happens...</p>
  
  <p>If the Event Aggregator is in Application and ItemView, why this code doesn't work ? :</p>
  
  <pre><code>var MessageItemView = Backbone.Marionette.ItemView.extend({
    template: "#messagesTPL",
    tagName: 'tr',
    className: 'messageItem',
    events : {
      'click': 'displayMessage'
    },
    displayMessage: function () {
      this.trigger('display:message');
    }
  });
  
  App.on('display:message', function () {
    console.log('display message !!!');
  });
  </code></pre>