0.9.2: Handle IPv6 in client_addr tween

ClientAddr tween was crashing when getting IPv6 addresses.

• Use IP version agnostic parsing.
• Fetch the list of CloudFlare's IPv6 IPs.

0.9.1: Bump release version

Can't release 0.9.0 to pypi as it was already created.

0.8.0: Add option to manually set maintenance mode

Use `python -m pyramid_heroku.maintenance` to manually set Heroku
maintenance mode.

Also, some cleanup and refactor.