Fix in-band registration ({register: true})

Due to some commit, in-band registration is broken in the current node-xmpp. The problem is that the `onStanza` method of `xmpp.Client` doesn't emit "stanza" events in `STATE_PREAUTH` phase, but that `doRegister` depends on such an event being fired to detect a registration reply IQ. I fixed this by emitting a new event type "stanza:preauth" in the `STATE_PREAUTH` and using that in `doRegister`.