[web] Security: verify HTTPS connections

Willie now verifies HTTPS connections when calling HTTPS-enabled web APIs.
It will use ca_certs from the config file,
and fallback to /etc/pki/tls/certs/ca-bundle.crt if ca_certs is missing from
config.

Verification of hostnames is done using backports.ssl_match_hostname.
This package is now mandatory to use willie.web, and is the most secure
and forward-compatible way (it's not needed in Python 3) to do hostname
validation.

Module authors specify verify_ssl = False in all web.py helper functions,
if they wish to disable https verification functionality.

Also change youtube.py to use gdata API over https, for easier testing
of this functionality.

Fixes #456