Reworked DefaultExpressionEngine to handle arbitrary nodes.

DefaultExpressionEngine was adapted to the modified ExpressionEngine interface.
It now uses a passed in NodeHandler to access the properties of the nodes it
has to process.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1563465 13f79535-47bb-0310-9956-ffa450edef68