Restore the certificate verification functionality in pyCardDAV.

The settings line in carddav.py deviates from the offical pyCardDAV
release, so restore that based on pyCardDAV release 0.7.0 to enable the
'verify' functionality.
Add a '--no-cert-verify' command line option.

This options skips SSL/TLS certificate verification. This comes in
handy when you want to transfer cards using a self signed certificate,
for example.