internal stanza handling was not recognized, causing service-unavailable messages on handled stanzas

the SaslHandler directly connected to the StreamParser to get its stanzas, while the service-unavailable stanzas are sent from the XmppClientConnection (which is also connected to the StreamParser)
the StreamParser's stanza event now checks whether the stanza was handled and otherwise raises an event (to which XmppClientConnection subscribes in order to send service-unavailable stanzas)
Merge pull request #9 from oli-obk/fix/service-unavailable