Tree with CheckBox nodes

Tree with CheckBox nodes is an extension for Ext.tree.TreeNodeUI to allow nodes to be 'checked'. Based on forum discussion and samples. Includes basic (read: crude) serialization to retrieve all of the checked nodes.

It also extends Ext.tree.MultiSelectionModel to allow multiple nodes to be selected and then checked by pressing the space bar. Checks cascade, so checking a child will check parents up the tree, and clearing a parent checkbox will clear all of it's children.

Additional changes include allowing CTRL click to deselect a selected node and shift click to select a contiguous range of nodes. Shift Click only works with nodes at the same depth beneath the same parent. You can click Blue and shift click on Orange to select both nodes and everything between, but you cannot shift click Horse and Red.