Version 2.4; add cpppo.history module
o Add timestamp class which manages conversion between UTC and local
  times and UNIX timestamps.
o Added pytz and tzlocal as optional requirements.  If tzlocal is found,
  we'll use it, otherwise we'll support simple Linux systems via TZ
  environment variable and/or /etc/timezone.  The cpppo.history module
  requires pytz to be installed for use.