Added fadeout to gh-popover

Closes #2868
- Uses jQuery's `.fadeOut` whenever a popover is closed.

Death to the ancient issues!
You could do this the other way around by observing the `open` property.  How would you feel about this:

```
close: function () {
  var open = this.get('open');
  if (!open) {
    // fadeOut....
  }
}.observe('open')