Added NodeTreeWalker class.

This class implements functionality for traversing nodes structures using a
ConfigurationNodeVisitor. Multiple traversal strategies are provided.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1570345 13f79535-47bb-0310-9956-ffa450edef68
InMemoryModel now uses a visitor for updating the parent mapping.

The iteration loop for traversing the nodes structure has been replaced by
NodeTreeWalker.walkBFS().

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1570346 13f79535-47bb-0310-9956-ffa450edef68