Add a blink comparator and pixel difference to image diffs

Find Pluto!

Try this for instance with the two images shown [here](https://www.luther.edu/astronomy/galleries/solar/pluto/) without looking at their animated gif first.

Viewing a commit diff is all about finding differences quickly. I think this pixel difference is a great way to quickly identify what has changed. For browsers that don't support the CSS for that yet, there's the good old blink comparator, which I find a bit more convenient than having to adjust opacity manually.

This is the last feature from me for this image diff, promised! :-) 

For details, see the commit message.

Tested on IE8, Safari 6.1.6 & 7.1, Chrome 38, FF 33.1 & FF 3.6.13 (yes, that old PC still has this ancient Firefox installed, and it also works with that!)