Feature Request: New Book button

[meelash]

Should be very simple to add a button to move the existing images into a folder named, "book 1", "book 2" etc. thereby emptying the images folder and resetting the pages count so it can match the book currently being scanned when running through multiple books in one sitting.

I'm quite close to getting pi-scan running on OS X in which case it will be easy for me to contribute to it and have a good debugging and testing environment, in which case I will do this and submit a pull request.