Put a pin on TouchImageView

The pin moves along the TouchImageView according to the drag/zoom moves. Its position in pixels can be obtained easily. If you change the type of pin, change the margins on ZoomablePinView.setMargins. This is the original pin:
![pin](https://f.cloud.github.com/assets/3913433/353494/28a8d5d8-a083-11e2-8658-a8d41c62b96f.png)

Furthermore, a bug is fixed, and the image is always centered after zooming out (original position).