Added function to close notification immediately from anywhere

Added function to close notification immediately from anywhere, this is using the className attribute. I created it without changing the code significantly, I tried to only add code.

Here's an example of how to use it:

// display notification that stays forever
alertify.log(<text>, <className>, 0);

// close notification from some event handler
$('#id').click(function () {
  alertify.closeNow(<className>);
});