Added 'hostname' parameter to serverHttp.listen()

Related to issue #642 - Add 'hostname' config option, for serverHttp.listen()
Mostly a security feature. If config.hostname is null, which is the usual case, it behaves as it used to. If `hostname: 'localhost'` is specified in the config, then http://localhost/ will work, but http://external_ip_address/ will not.