Allow parent, such as ViewPager, to scroll if no Drawable loaded into PhotoView

Hello Chris,

I'm not sure if this is desired or not, but I added an if statement in onDrag() in PhotoViewAttacher that allows parent scrolling if no image has been loaded into PhotoView. 

This is useful if the user wants to continue scrolling items in a ViewPager even though the current item has not been loaded.

Thanks,

Igor