[ssl] add optional support for certifi

Certifi is a collection of root ca certificates, extracted from requests project.
It's much easier to use it, instead of bringing your own certificate collection (or trying to find system's collection).
It should help this project with:
- handling ssl errors originated by outdated root ca collection
- handling ssl errors because collection file was not found on the system
- supporting wider range of platforms out of the box (os x for example)

I wrote this after trying to run willie on OS X without success.