Fix a security issue where SSL certificates on HTTPS-hosted CAS servers were not validated.  Also:

- Migrated from using urllib to requests, as it makes the SSL handling much easier.
- Migrated from distutils to setuptools in order to facilitate automatic installation of requests.
- Updated example proxy_authentication to use requests.
- Added manual certificate selection for CAS server.
- Added client certificate support for CAS server.
- Added hashbang to setup.py so that it is executable on *NIX systems.