Added ReferenceTracker class.

This class manages reference objects that have been assigned to the nodes of a
model. The references are kept up-to-date, even if nodes in the model are
replaced during update operations.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1585183 13f79535-47bb-0310-9956-ffa450edef68