use similar interface like node-xmpp exposes

I don't understand why `require('node-xmpp')` differs from `require('node-xmpp-core')`. They dont expose the same interface. node-xmpp returns an object containing Stanza, Message, Iq, etc while node-xmpp-core returns an object with the key stanza containing an object containing Stanza, Message, Iq, etc.
This PR corrects this.