Added the method getModel() to HierarchicalConfiguration interface.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1581042 13f79535-47bb-0310-9956-ffa450edef68
Reworked HierarchicalConfigurationXMLReader to be compatible with immutable nodes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1581044 13f79535-47bb-0310-9956-ffa450edef68