Reworked ConfigurationNodeIteratorAttribute to use a NodeHandler.

Attributes can now also be processed with the help of a NodeHandler.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1565795 13f79535-47bb-0310-9956-ffa450edef68
Added a special NodePointer implementation for attributes.

This implementation wraps a single attribute of a configuration node which is
represented by a QueryResult object.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1565796 13f79535-47bb-0310-9956-ffa450edef68