Simplify/improve the visual diff

 * Move the calculations to the render funcion
 * Stop automatically `$sce.trust()`-ing the HTML code.
   (So now `$sanitize` is invoked automatically.)
   This also removes the need to use the `$sce` module,
   or to set the `trustedDIffHTML` flag, simplifying the code.
 * Use the `objectEquality` flag when `$watch()`ing the targets,
   so that we pick up changes in both the number and the contents
   of the targets