Use PointerEvents only if Gesture API is available

Closes #17

http://msdn.microsoft.com/en-us/library/ie/hh673557(v=vs.85).aspx#gesture_object_events

>In addition to pointer events, Windows 8 has the ability to recognize
complex interactions called gestures (such as pinch, swipe, and so on)
in a consistent way across applications.
>
>The APIs described in this section are not supported in Windows 7.