Adding SSL/TLS support to mosca

Secure mosca server can now be instantiated by including a secure key and signed cert in the settings passed to the server:

var moscaSettings = function() {
  return {
    port: nextPort(),
    logger: {
      childOf: globalLogger,
      level: 60
    },
    secure : { 
      keyPath: SECURE_KEY,
      certPath: SECURE_CERT
    }
  };
};

Test cases have been added, but are currently being skipped.  All of them pass except for the ones that are using MQTT ascoltatore as a backend. When this happens we try to connect a non-secure client to the now secure server so the test fails. 

Matteo - I propose refactoring MQTT createClient to just take a option dictionary, where the secure key/cert can be passed in similar to mosca.  This should allow the failing tests to pass. What do you think?