Support settings arguments per tabset.

In http://drupal.org/node/137659 a developer wants to add custom effects to tabs. The easiest way would be to pass an array of settings with the form. Approach:

* devs can add #settings key to tabset $form element.
* this is passed to browser as a Drupal.settings object
* tabs library reads them in when adding the tabs

I don't have time right now to implement this, I'd welcome a patch, or I'll try to get to it when I can.