Added NodeModelSupport interface.

This interface allows access to a NodeModel. It will be implemented by
hierarchical configurations. Having this separate interface allows for better
modularization. Components only interested in a NodeModel need not bother
with a full-blown hierarchical configuration.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587022 13f79535-47bb-0310-9956-ffa450edef68