Add sortable.js for sortable tables in output.

Add sortable.js for sortable tables in output.

This adds the sortable table JavaScript from
http://yoast.com/articles/sortable-table/ (version 1.5.7) and some
helper images from the same site.

[hoxu@users.sf.net: The license linked from sortable.js and the one linked from
yoast.com (to opensource.org) differ a bit (the middle sentence about
copyright/permissions notice is not in the first), so I'm including a verbatim
copy-paste from http://www.kryogenix.org/code/browser/licence.html here:
-----
Copyright (c) 1997-date Stuart Langridge

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
of the Software, and to permit persons to whom the Software is furnished to do
so, subject to the following conditions:

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
-----
With the addition that Joost De Valk has improved the script.]
Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>