Count with offSetScroll in calculateLimit

When dragging a element inside a container && offSetParent is scrolled,
the position of dragged element is wrongly calculated.

Example of buggy behavior: http://jsfiddle.net/F9Fc9/
Merge pull request #1266 from SergioCrisostomo/fix1010-drag-offsetScroll

Count with offsetScroll when calculateLimit()
Fix scroll miss-placement while dragging

When dragging and scrolling happen at same time, the dragged element
does not follow the scroll and gets "left behind". This commit/PR fixes
that. The old behavior is still optional.
DRY improvements