Implemented a thread-safe update mechanism.

Updates are now done using the compareAndSet() method of the atomic reference
storing the current data of the model.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1571046 13f79535-47bb-0310-9956-ffa450edef68
Changed the structure of the personae tree.

Added an additional child node below the works. This is useful for testing
clearProperty() operations because these nodes have a value.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1571047 13f79535-47bb-0310-9956-ffa450edef68