Make QRCodeReaderViewController an open class to allow subclassing

[nickbit]

For my case I need to hide the navigation bar when QRCodeReaderViewController is pushed in the navigation controller. This cannot be done without overriding the functions viewWillAppear and viewWillDisappear of UIViewController. Other tweaks could be also not be possible without overriding the class.

So a simple change in the source code can solve these problems easily, just make the public class QRCodeReaderViewController open.