Add a blink comparator and pixel difference to image diffs

Pixel difference uses CSS mix-blend-mode, which is supported currently
only on Firefox >= 32 and on Safari >= 7.1. Implementation is behind a
Javascript feature test.

For other browsers, there's a blink comparator.

Code changes:

* ImageDiffHandler now takes the page it's used on as argument. We need
  that to get labels. DOM generated is a
  little bit different (new controls).
  
* Diff pages adapted to new constructor of ImageDiffHandler.

* CSS and Javascript changes implementing the new controls, making use
  of two new static image resources. Since I felt that the new controls
  deserved tooltips, I also gave the opacity slider a tooltip: changed
  to <a>, and slider handle changed from <div> to <span>. CSS ensures
  everything still displays the same (basically display:inline-block).
  
* Supplied messages for English, French, and German for the new
  tooltips.

Tested on IE8, Safari 6.1.6 & 7.1, Chrome 38, FF 33.1 & FF 3.6.13
Merged #222 "Add a blink comparator and pixel difference to image diffs"