BaseHierarchicalConfiguration now extends AbstractHierarchicalConfiguration.

Removed all code inherited from the base class. Also, the unit test was
stripped down to test only specific functionality. The tests fail currently,
however, at least the code compiles.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1573345 13f79535-47bb-0310-9956-ffa450edef68