title: Add (open/close) button to activity for (opening/closing) the navigation drawer
body: <p>I am currently learning about the Navigation Drawer from the android site, and I am using their example <a href="http://developer.android.com/training/implementing-navigation/nav-drawer.html">http://developer.android.com/training/implementing-navigation/nav-drawer.html</a></p>
  
  <p>What I want is to add a button in the <code>MainActivity</code> which would be able to open the <code>NavigationDrawer</code>. I need to do it programmatically, not in XML. How can I do that?</p>