Added an event - background_drag

[jesinwp]

This event triggers when dragging the background image and contains the X and Y background-position values of the background-image.

Usage:

$('#draggable_bg').background_drag(function(e){
    $('p#bg_coordinates').text(e.posX + 'px x ' + e.posY + 'px' );
});

$('#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/