InMemoryNodeModel now supports reference objects that can be assigned to nodes.

This is required for some concrete configuration implementations which needs to
store additional data for their nodes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1585184 13f79535-47bb-0310-9956-ffa450edef68
Reworked XMLConfiguration to operate on immutable nodes.

The major part of the functionality for loading and saving XML documents is
working, but there is still a bunch of tests failing.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1585185 13f79535-47bb-0310-9956-ffa450edef68