Started rework of INIConfiguration to be compatible with immutable nodes.

Loading the configuration now creates a hierarchy of immutable nodes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580597 13f79535-47bb-0310-9956-ffa450edef68
Changed argument type of NodeStructureHelper.nodeForKey().

The method can accept a more general NodeHandler<ImmutableNode>; this interface
also has a method for querying the root node.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580599 13f79535-47bb-0310-9956-ffa450edef68