Configured grunt-usemin to replace images added in templates by its revved version.

* Updated grunt-usemin to version 2, allowing addition of custon patterns to
  replace the images.

* Included pattern in Gruntfile, to replace some images added to
  templates.

  TODO: replace the manual patterns by a regex.

* Renamed the usemin.options.dirs to usemin.option.assetsDirs, once it
  was changed in version 2.
Configured grunt-contrib-cssmin to minify the css used by colorbox package.