Added swal.cancelAlert() method that can be called globally to close the...

... current modal with close animation.

Relevant Issue: #200 

This allows SweetAlerts to be used as a kind of busy "spinner" for AJAX request.

```
swal({
  title: "Searching...",
  text: "Click to cancel your search.",
  type: "info",
  showCancelButton: false,
  confirmButtonColor: "#DD6B55",
  confirmButtonText: "Cancel search",
  closeOnConfirm: true
});

$.ajax({
  url: "test.html",
  context: document.body
}).done(function() {
  // do something here 
  swal.cancelAlert()
});
```