Added NodeHandler interface.

In future we are going to support different types of hierarchical configuration
nodes. The purpose of the NodeHanlder interface is allowing (read-only) access
to the data of a specific node type.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1561334 13f79535-47bb-0310-9956-ffa450edef68
Added initial implementation of ImmutableNode.

This class provides read-only access to properties needed for a node in a
hierarchical configuration.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1561335 13f79535-47bb-0310-9956-ffa450edef68