Image diffs

Ticket 88: https://dev.gitblit.com/tickets/gitblit.git/88

Based on Lea Verou's pure CSS slider:
http://lea.verou.me/2014/07/image-comparison-slider-with-pure-css/

* Add a callback interface, pass it through DiffUtils to the
  GitBlitDiffFormatter. Is needed because the rendering needs access
  to the repositoryName and other things that are known only at higher
  levels.
  
* New class ImageDiffHandler responsible for rendering an image diff.
  Called for all binary diffs, doesn't do anything if it's not an
  image. HTML is generated via JSoup: no worries about forgetting to
  close a tag, not about HTML escaping, nor about XSS.
  
* The 3 diff pages set up such an ImageDIffHandler and pass it along.

* CSS changes: from Lea Verou, with some minor improvements.

I think in the long run there'll be no way around rewriting the
HTML diff formatter from scratch, not using the standard JGit
DiffFormatter at all.