Updated script to prevent jumping on initial scroll & after scrollbar drag

Updated to fix following bugs:
If $.smoothWheel() was called on an element after it had been scrolled,
or if the scroll was set by dragging the scrollbar, scrollTop would
jump to 0 or its previous position, respectively, upon use of the
scrollWheel.
Merge pull request #1 from JRGould/master

Updated to prevent scroll jump on init & after scrollbar drag