Added tests for creating sub configurations.

It has to be ensured that the synchronizer is called before the sub
configurations are otained. Otherwise, the combined configuration is not yet
initialized.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587017 13f79535-47bb-0310-9956-ffa450edef68
Added FindNodeVisitor class.

This is a node visitor implementation which scans a hierarchy of nodes for a
given target node. It used by CombinedConfiguration to determine the source for
a given key.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587018 13f79535-47bb-0310-9956-ffa450edef68