Add possibility to access the click source from callbacks

It is sometimes useful to be able to access the click source element from callbacks : 

```js
<script>
  $(document).ready(function () {
    (function() {
      $('#slider').slideReveal({
        trigger: $('.trigger'),
        show: function(event, element) {
          console.log(element.data('url'));
        }
      });
    })();
  });
</script>
```