Added jQuery stuff. It works!

I did the following steps (roughly):

  - Added a simple foreach-based table generation code in
  IndigoPlateau#print_ranking.
  - Removed the json_encode() call from jsonify_players() and changed
  its name to create_players().
  - Added wp_enqueue_script() calls to indigo-plateau.php to insert
  jQuery UI's accordion and datepicker. autocomplete will be inserted
  soon.
  - Added the JavaScript code necessary to create a datepicker and the
  accordion in the admin panel code. Of course, this won't stay here.

So, I still need to refactor most of the methods from IndigoPlateau and
the admin panel's code -- it could use some ajax requests. Ah, and I
need to include a JS file in this plugin, initializing everything &c.