Add circle image deskewing

[hadess]

Because some CDs are effectively mirrors, I've taken photos at an angle, using a backing white sheet to capture that mirror part cleanly. But this means that I can't use MobyCairo, as it doesn't correct the angle, and so can't detect the circle for cropping afterwards.

This should be correctable using "homography" as described here, in an additional step: https://stackoverflow.com/questions/25724923/opencv-how-do-i-implement-keystone-correction

Test Image