corrected dragging service enable draging in all directions using abs

Hi (first time I create a pull request on github so if I did something wrong sorry ...), 

I realized using the flowchart that dragging wouldn't start when moving the mouse upward or to the left. The reason is  evt.pageX - x or evt.pageY - y would return negative negative value which do not trigger the test (more than threshold).  So I changed this using Math.Abs(evt.pageX - x) so it triggers dragging whatever direction you go. 

Very nice article and project thanks !