Added an event - background_drag

This event triggers when dragging the background image and contains the X and Y `background-position` values of the `background-image`.

__Usage:__
```js
$('#draggable_bg').background_drag(function(e){
	$('p#bg_coordinates').text(e.posX + 'px x ' + e.posY + 'px' );
});
```

```js
$('#draggable_bg').on('background_drag', function(e){
	$('p#bg_coordinates').text(e.posX + 'px x ' + e.posY + 'px' );
});
```

JSFiddle demo: http://fiddle.jshell.net/jesin/AL8hX/show/light/