Added new AbstractHierarchicalConfiguration base class.

This class provides basic functionality of a hierarchical configuration based
on a NodeModel. The idea is that by providing alternative implementations of
NodeModel, arbitrary hierarchical data structures can be integrated with this
class.

The current implementation is incomplete, there are still some failing tests.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1573219 13f79535-47bb-0310-9956-ffa450edef68