Added parameters to open and close callbacks

afterOpen and afterClose callbacks now have access to the current
target jQuery object.  This is useful if you need to know which image was clicked
to open the gallery and which image was viewed when users decided to
close the gallery, respectively.

The minified version of the JavaScript file (``jquery.swipebox.min.js``) 
has been created using the Grunt task supplied in the project.

Sample usage:
```javascript
$(".swipebox").swipebox({
  afterClose: function( imageNode ) {
    if ( imageNode === undefined ) {
      return;
    }
    // your code goes here
    // ...
  }
});
```