InMemoryNodeModel now supports an addProperty() operation.

A new internal helper class ModelTransaction was introduced which combines a
number of updates on nodes. In this class add operations have been implemented.
Other update operations are still pending.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1568649 13f79535-47bb-0310-9956-ffa450edef68