Fix scroll miss-placement while dragging

closes #1259

When dragging and scrolling happen at same time, the dragged element
does not follow the scroll and gets "left behind". 

Example of the problem: http://jsfiddle.net/hFnXq/2/
Example fixed: http://jsfiddle.net/hFnXq/

This commit/PR adds a new option and defaults to new behavior.