Update to work with Python 3

[criptych]

Python 2 is at end-of-life (today!) and there are (just a few) problems running this script under Python 3. This PR makes most of the changes recommended by 2to3 (replacing urllib2 and adding parens around print arguments) as well as the following style and bugfix changes:

• Replace optparse (deprecated since 2.7/3.2) with argparse
• Replace arguments taking a bool parameter with 'store_true'
• Use self.<method> instead of Class.<method>(self, ...) in PhpSerializer
• Fix bytes vs string usage (especially for loading signature database)