Add support for non-zipped archives

[nicorikken]

The Gnupg-users list has archives, but as of April 2014 the archives are no longer zipped. http://lists.gnupg.org/pipermail/gnupg-users/ Add support for non-zipped files as well.
Current error I get for the previously mentioned link:

$ ./mailman-rss.py -c 1 https://lists.gnupg.org/pipermail/gnupg-users/
Traceback (most recent call last):
  File "./mailman-rss.py", line 212, in <module>
    main()
  File "./mailman-rss.py", line 193, in main
    mbox = archive.get_month(month_url)
  File "./mailman-rss.py", line 70, in get_month
    data_gz = fetch(url)
  File "./mailman-rss.py", line 48, in fetch
    fd = urllib2.urlopen(url)
  File "/usr/lib/python2.7/urllib2.py", line 154, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python2.7/urllib2.py", line 435, in open
    response = meth(req, response)
  File "/usr/lib/python2.7/urllib2.py", line 548, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python2.7/urllib2.py", line 473, in error
    return self._call_chain(*args)
  File "/usr/lib/python2.7/urllib2.py", line 407, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.7/urllib2.py", line 556, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 404: Not Found