Added NodeSelector class.

This class allows selecting a specific node based on a single key or a chain
of keys.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1575753 13f79535-47bb-0310-9956-ffa450edef68
Moved some functionality related to mock resolvers to NodeStructureHelper.

Working with mock resolvers can be useful for multiple test classes. Therefore,
it makes sense to have this functionality at a central location.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1575754 13f79535-47bb-0310-9956-ffa450edef68