Introduces $.fn.txpSortable().

First take on the sortable widget integration. This method
creates a sortable widget and syncs updated orders with
the server. For now only supports single lists.

See issue #356

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5168 2fea6a4d-a838-0410-917b-93a53c48e9d2