switch positioning during drag to use CSS transform + RAF

The dragged element is now positioned using transform: translate(x, y)
rather than left: x, top: y.

Position updates are now done via requestAnimationFrame rather than
immediately on every pointer movement event.
pass $el as argument to positionElement function
remove transform positioning (at least for now)
add the getStartPosition option
rename long_press option to longPress
implement 'handle' option for drag handle(s)
Add a changes file
bump version for release and update demo URL