Merge pull request #1 from BarAgent/od-compatibility

Work around issue #424. Add test application.
Handles <object> in the classes-to-visit list.
Works around problem with plists' remove-property!.
Adds a note about <string> being a collection.
Adds sufficient keyword defaults to run a collection-recursive visitor on a collection directly.
Omits setter: when recursing into immutable collections.