Added ReferenceNodeHandler interface.

This interface allows access to reference objects which can be associated with
the nodes stored in a NodeModel.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1585181 13f79535-47bb-0310-9956-ffa450edef68
Added mock support for resolve add key operations.

The method for preparing a resolver mock to support add keys was moved from
TestInMemoryNodeModelTrackedNodes to NodeStructureHelper. It is useful for
other tests, too.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1585182 13f79535-47bb-0310-9956-ffa450edef68