Allow nodes (and subgraphs) to be detached from the session and reattached to another session

This was inspired by a question by Gert-Jan on StackOverflow.

It is not possible to read properties from a node once the session has been closed. It would be useful to provide a way for the node to be detached from the session (prior to the session being closed), at which point all of the properties would be stored locally within the node. It might also be useful to allow the node to be reattached to another session, at which point the node object should reflect the current state of that node within the new session.

This would actually be quite straightforward if the "detach()" and "attach()" methods returned a Node reference (which may or may not be the same object upon which the method was called). But that might make it more difficult to use.