Added optional fetch of thumbnail photo

For displaying the same cropped image the Address Book shows. It's probably also faster since the image is smaller.

Got it from here. http://stackoverflow.com/questions/8972140/getting-the-cropped-version-of-a-picture-from-the-address-book

On the demo, on `self.photoView.image = contact.photoThumb` just switch photo with photoThumb and the images on the list will change to the Address Book thumb ones.