Add pinch gesture recognizer
Add pinch gesture recognizer

It still has some issues on transform/frame and behaves bad when zoom in, drag, and moved to default position.

hiragram/YHImageViewer,hiragram/YHImageViewer,hiragram/YHImageViewer,wtshm/YHImageViewer,wtshm/YHImageViewer,wtshm/YHImageViewer,wtshm/YHImageViewer,hiragram/YHImageViewer