Add PureXML.Model.TreeNode, a reference-type document tree that knows its
parent and siblings. Navigation goes up and sideways (parent, nextSibling,
previousSibling, ancestors, root, elementChildren, stringValue) and editing
is in place (append, insert before/after, removeFromParent, replace, copy).
Children are held strongly and the parent weakly; attaching a node detaches
it from any prior parent and a node can never become its own ancestor.
parseTree builds one from XML; TreeNode.node converts back for serialization.
Closes#31
