Rfc2120

Closes #101

I've updated the code to remove authzid unless user provides an 'actAs' key in the config.  

Have tested digest-md5 login against Tigase and Prosody without 'actAs' both authenticate ok.  You can also provide the 'actAs' key and both servers will continue to authenticate.

```javascript
var client = new xmpp.Client({
    jid: 'admin@host.name', 
    password: 'admin',
    xmppHost: 'host.name',
    actAs: "romeo"
});
```

I've also fixed an issue with ```Mechanism``` and ```authAttrs()``` method (needed to be defined on the prototype).

For both Prosody and Tigase you can't actually behave as another user as the data is ignored at present (I'm told Tigase may be implementing this soon).

Cheers, Lloyd.