Handle touch related events (W3C and Windows touch events).

The touch events now triggers:
   ol.MapBrowserEvent.EventType.TOUCHSTART,
   ol.MapBrowserEvent.EventType.TOUCHMOVE,
   ol.MapBrowserEvent.EventType.TOUCHEND

The new ol.MapBrowserEvent.targetTouches property contains a list of
touch events for touch points that are still in contact with the touch
surface and whose touchstart event occurred inside the same target
element as the current target element. This list exists to unify the
W3C and Windows touch api.

FIXME:
 * only compute ol.MapBrowserEvent.targetTouches on touchinteraction ?.
 * don't compute ol.MapBrowserEvent.targetTouches on W3C compatible devices.