TreeDropdown-like field that gives you a tree of items including an empty field, using ajax.

Author: Marijn Kampf www.exadium.com
Date: 24 Nov 2009
Version: 2.2
Revision date: 9 May 2011
Changes: Fixed ajax sub tree request
Revision date: 8 October 2010
Changes: Changed $this->postTree to OptionalTreeDropdownField::$postTree to avoid Undefined error
Revision date: 17 June 2010
Changes: Updated to work with SilverStripe 2.4, tree function added.