Fix Issue #2: Include pad-based fuzz, for message authentication.

* onetime
  (Pad.__init__): New self._begun flag variable.
  (Pad.convert): Take new format_version argument.
    Set the self._begun flag.
  (Pad.read): Remove; self._make_fuzz will handle this now.
  (Pad._make_fuzz): New function.
  (PadEncoder.__init__, PadDecoder.__init__): Remove self._used flag.
  (PadEncoder.encode): Remove fuzz code, as Pad handles this now.
  (PadDecoder.decode): Same, but pass format_version where needed.