Added (optional) ttBranch data attribute.

This attribute is useful for idenitfying a node as a potential branch
node even though it has no children yet.
Remove redundant jQuery wrap of row
Made appropriate plugin 'methods' chainable
Added loadBranch plugin function. Use this function to add new rows (nodes) to the tree.
Added unloadBranch function for (recursively) removing child nodes from
a parent node.

Note that this will remove the table rows from the DOM and remove all
references to the nodes from the Tree object.