support jQuery UI pre 1.9

jQuery UI 1.9 adds _super() and _superApply(); this update removes the
two instances of _super() from selectableScroll.js, so that the plugin
works with older versions of jQuery UI (tested with 1.8.23)
added useParentForScrolling option

Added an option "useParentForScrolling" to scrolling calculations to be
done with the parent of the widget, for situations where you want to use
a wrapper div. (See
http://stackoverflow.com/questions/2330773/jquery-ui-selectable-plugin-make-scroll-bar-not-selectable-when-div-overflows
for an example. This is something you might want to do if you don't want
clicks on the scrollbar to be treated as selections in the selectable.)