Fixed Draggable on Safari and mobile Safari

webkitMovementX and webkitMovementY are not currently supported in the version of webkit that ships with Safari (OS X 10.9.2) and mobile Safari (iOS 7.1).

Reverted the position delta tracking (as seen in this commit https://github.com/jimray/Framer/commit/5fce26486cf2149e5a9c1027e0b5fccd2c7cb04f ) to the manual calculation instead of relying on the webkitMovement properties.