Fix panning on iOS devices

This may break panning on Android devices, we'll have to try and see.
The W3C TouchEvent API[0] doesn't include the pageX and pageY attributes
that (PhoneGap on) iOS seems to use instead of the various TouchList
attributes, so we may have to behave differently on different devices.

0: http://www.w3.org/TR/2013/REC-touch-events-20131010/#touchevent-interface