Reworked XPathExpressionEngine.

The class now correctly implements the extended ExpressionEngine interface.
This means that XPATH expressions can be executed on arbitrary node models
for which a NodeHandler implementation exists.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1565802 13f79535-47bb-0310-9956-ffa450edef68
Adapted test class for ConfigurationNodePointerFactory.

Now a node structure of ImmutableNode objects is queried.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1565803 13f79535-47bb-0310-9956-ffa450edef68