Added 'dissimilarity-threshold 1' command line option to ImageMagick...

Added 'dissimilarity-threshold 1' command line option to ImageMagick Compare utility to force diff even if the images are 100% dissimilar

This patch adds a command line option that prevents 'compare' from refusing to create the diff if the images are too dissimilar. I don't think it is necessary to make this configurable as this is the behaviour Wraith (and the users) expect.