Implement shouldStopEvent in DragBox interaction

This fixes a bug [reported](https://groups.google.com/d/msg/ol3-dev/WpHqjbcq7qU/51SFp0NjD2EJ) on the mailing list where the map is panned while boxes are drawn.

Please review.

Note: the shouldStopEvent function was introduced because of the PinchZoom and PinchRotate interactions that need to work together, that is the PinchZoom interaction should not stop event and prevent the PinchRotate interaction to trigger (and vice-versa). We could probably remove the shouldStopEvent hack by combining these two interactions into one.