Allow serving content directly over https to simplify development environment (second take)

When developing/testing locally it's more convenient to let fxa-auth-server directly serve https.

This is what this pull request allows.  It is controlled by a useHttps config key, to turn it on/off, and two keyPath and certPath entries pointing to the pem key and certificate.

It's the same code as #727, except that keys have been renamed to camelCase.