Add a 'drupal_add_tabledrag' example

I've got a working 'sorting' tabledrag example and a 95% complete 'parent/child' tabledrag example set up in a sandbox at http://drupal.org/sandbox/jthorson/1440578 (under tag 7.x-1.0). The parent/child example does not display the parent/child relationships when the table first loads, but the dragging/saving functionality is working and complete. (If someone could point me in the right direction to get the parent/child relationships to display on first load, it would be greatly appreciated.

Both of the above could use tests ... but I'm not sure how to test that a client-side javascript functionality is working properly?

I've also began building out a 'group' sorting example and the filenames for a combined 'group+parent/child+sorting' example in the 7.x-1.x branch, but these are currently just copies of the code from the 'simple' case, and not really close to working. If anyone wanted to update them, that would be a big help as well.