Improve SSL default settings and functionality.

- Don't set ssl_version unless it's provided. Use the ruby default instead.
- Enable SSL peer verification by default.
- Allow setting SSL cipher suites, and override the ruby defaults if they seem to be insecure.
- Expose setting an SSL certificate store.
- Set a default certificate store using the system roots, even on Windows.
- Don't use verify_callback because it's not supported on jruby.