title: Access a global variable from any app controller
body: <p>I need to use a global variable (user context, available from all the code)
  I have read some posts regarding this topic but I do not have a clear answer.</p>
  
  <pre><code>App = Ember.Application.create({
    LOG_TRANSITIONS: true,
    currentUser: null
  });
  </code></pre>
  
  <ol>
  <li>Is it a good practice to set the currentUser global variable in the App object ?</li>
  <li>How to update and access the currentUser property from all the controllers used in the app ?</li>
  </ol>