Add support for tray click event on os x

As @trevorlinton doesn't created a pull request for this for the last 4 month I am doing it now. This will fix Issue #510. 

Simple example:  
```html
<html>
<body>
  <div>Click on the tray icon</div>
  <script>
    // Load library
    var gui = require('nw.gui');

    // Reference to window and tray
    var tray = new gui.Tray({ icon: 'icon.png' });

    // Show window and remove tray when clicked
    tray.on('click', function() {
      alert("tray clicked");
    });
  </script>
</body>
</html>
```