Add digitizer/pen twist

[patrickhlauke]

Closes #25

[patrickhlauke]

@dfleck one minor question: you mentioned that the value is 0-360. https://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.input.pointerpointproperties.twist?f=255&MSPPError=-2147217396 mentions 0.0 - 359.0. I assume to be precise, it's 0.0 - 359.9999999999999999 (or whatever the precision is). does it make sense to just say 0-360 for simplicity's sake?

[RByers]

Should we just reference the USB HID Usage? I.e. Usage ID 0x41? Though I still don't see any definition in that spec of the allowable values. Still saying simply 0-360 seems reasonable to me (we can write it [0-360) if we want to be precise that it's exclusive of 360).

[patrickhlauke]

Agreed in today's call https://www.w3.org/2016/06/08-pointerevents-minutes.html to merge this as is - as the property is of type long, [0,359] is appropriate and less confusing than [0,360)