Added meaningful ToString() to all system messages

[Horusiath]

cc #72

Hope this will close the case.

[Aaronontheweb]

@Horusiath I doubt these changes were the reason why the ThrottlerTransportAdapterSpec failed, so I queued up a new version of this build to run just to verify. Thanks for taking care of this!

[Aaronontheweb]

Wow, that's not at all what I expected... Somehow these changes did effect these three tests consistently, even on different build servers. What could have caused that?

[rogeralsing]

Found the issue..
It throws Null ref exception in Identify.ToString

  public override string ToString()
        {
            return "<Identify>: " + MessageId.ToString();
        }

Remove the ToString on messageId and it is fine