Add tls_cert, tls_key and tls_ca_cert configuration options

See: ARAKOON-428
See: http://jira.incubaid.com/browse/ARAKOON-428
CI: Install libssl-dev & OCaml 'ssl' package
Include 'lwt.ssl' dependency

See: ARAKOON-428
See: http://jira.incubaid.com/browse/ARAKOON-428
Add TLS support to __open_connection

See: ARAKOON-428
See: http://jira.incubaid.com/browse/ARAKOON-428
Implement node-to-node TLS connectivity

See: ARAKOON-428
See: http://jira.incubaid.com/browse/ARAKOON-428
Add mkcerts.sh

Note this utility script should not be used to generate certificates
etc. for a real production setup. It should be considered insecure, has
not been audited,...
Add basic TLS validation test
Add client service TLS support

See: ARAKOON-428
See: http://jira.incubaid.com/browse/ARAKOON-428
Accept TLS connections without certificate

Certificate checking should be performed at some other level (e.g.
by using `Ssl.Verify_fail_if_no_peer_cert`).
Implement TLS client support for CLI actions
Doc: Add TLS setup instructions
Tests: Use client TLS connections in test_basic_tls
Merge ARAKOON-428 into 1.7 (#172)