Added an option to Invert the overlay image.

[mbuckley]

Inversing the image shows the differences as color flashes. Areas that match perfectly will remain gray.

I've attached a test image of the getfirebug homepage as well as a sample screenshot where I've moved the logo over a few pixels to illustrate what happens when there is a difference between the page and the overlay :)

[mbuckley]

If the image is completely gray that means that the web dev is perfect to original design

[janodvarko]

Looks great! Honza