Reworked PropertyListConfiguration to operate on immutable nodes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588683 13f79535-47bb-0310-9956-ffa450edef68
Reworked PropertyListParser to operate on immutable nodes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588684 13f79535-47bb-0310-9956-ffa450edef68