Support verification of client-side SSL certs

Added an explicit option for SSL's `verify_mode`. In the event that a CA file
or CA path are passed, default to expecting clients to have signed certs. If
nothing is provided, use the preexisting behavior of not verifying clients.
Test with signed client certificates
Test with signed server certs, too
Test for failure with unsigned client certificates
Merge pull request #113 from stouset/client-ssl-certificates

Client SSL Certificates