Python 2.4 compatiblity

- Discovered the CronExpression code was only a few lines away from
  Python 2.4 copatibility and tweaked it accordingly.
Prepared setup.py for PyPI upload

- The setup.py file has been modified to include classifier information,
  and if Pandoc is available, it will convert the README to REstructured
  Text so that it can be shown on the PyPI page.