Implemented configurationAt() in BaseHierarchicalConfiguration.

The method now either creates a BaseHierarchicalConfiguration initialized with
the selected root node or a SubnodeConfiguration with a corresponding
NodeSelector. Adapted test classes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578582 13f79535-47bb-0310-9956-ffa450edef68
Implemented configurationsAt() in BaseHierarchicalConfiguration.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578583 13f79535-47bb-0310-9956-ffa450edef68