Log Message:
Update to 2.18:

2.18  Aug 15 2007
      - Non-unique key attribute values now trigger a warning (or a fatal
        error in strict mode) rather than silently discarding data (patch
        from Daniel Baysinger)

2.17  Aug 02 2007
      - Added parse_string(), parse_file() and parse_fh() methods
      - Added default_config_file(), and build_simple_tree() hook methods
      - Tweak to implementation of exporting (patch from Stuart Moore)
      - Documented hook methods
      - Fixed test suite race condition (RT#28603 from Andreas J. König)