Draggable stack not working (found problem and fixed)

Hi,

I tryed to use the new 1.8 draggable in a project and found that the stack option is not working and fixed it. Hope it helps:

Line 769 or around looks like this:

var group = $.makeArray($(o.stack)).sort(function(a,b) {

and should be, in my opinion, like this:

var group = $.makeArray($(o.stack.group)).sort(function(a,b) {

and now

stack: {group: '.post-it', min: 1} 

works as expected (by me at least) on all elements with a "post-it" class.