Added NodeUpdateData class.

Similar to NodeAddData, this class stores all information to perform an update
operation on a node model.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1568646 13f79535-47bb-0310-9956-ffa450edef68
Added NodeKeyResolver interface.

This interface allows a concrete node model implementation to interact with an
ExpressionEngine providing some more functionality than the engine itself.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1568647 13f79535-47bb-0310-9956-ffa450edef68