joesimms: added draft documentation for hierarchical data support, and docs on how to use the tree interface, in particular the NestedSet implementation

joesimms: added support for many roots for one table, added another example illustrating this, see EXAMPLE2.tree.php, also changed toString to __toString in getPath()

joesimms: initial draft for tree support. NestedSet support included, placeholders for other popular implementations also included. Read the README.tree file for more information and changelog to core files. Modified core files have also been included in this commit. hope it works and you like it !