Implemented childConfigurationsAt() in BaseHierarchicalConfiguration.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578584 13f79535-47bb-0310-9956-ffa450edef68
Added hooks for the creation of a sub configuration.

There are new protected methods for defining a sub configuration's parent model
and its NodeSelector.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578585 13f79535-47bb-0310-9956-ffa450edef68