Update PopUpManager.as

hi, I found "root setter" function has a issue.
...
var popUp:DisplayObject = popUps[i];
var overlay:DisplayObject = DisplayObject(POPUP_TO_OVERLAY[i]);
...

the overlay always return null, I think because 'POPUP_TO_OVERLAY[i]' should be 'POPUP_TO_OVERLAY[popUp]';
Merge pull request #692 from lynxerzhang/patch-1

Fixed issue where changing PopUpManager root failed to transfer overlays.