On Saturday at noon, Justin Herman Plaza was sun-drenched and buzzing with the activity of particularly open-minded crowd, and passersby were beginning to take notice. "What the hell are all these people doing half-naked and shit?" one man cried.He was obviously not familiar with the way things are here in San Francisco, where bare and body-painted cyclists have pedaled to Golden Gate Park and back once a year, for the past six years, as part of the larger international World Nake