Improve performance by changing binding of mousemoveDocument to $menu

Was  `$(document).mousemove(mousemoveDocument);`
Which means that *every* mouse-move on the document would of trigger this callback...
which would be very un-efficient (especially on medium to large pages with lots of DOM elements..

So I suggest changing it to:  `$menu.mousemove(mousemoveDocument);` (I tried and it still works!) 

Perhaps add the binding of `mousemoveDocument` as an `option` where the user can define a selector..