Add StreamError stanza and a stream_error event.

Note that the stream may automatically attempt to
reconnect when a stream error is received.

sleekxmpp/stanza/__init__.py

@@ -8,6 +8,7 @@
 
 
 from sleekxmpp.stanza.error import Error
+from sleekxmpp.stanza.stream_error import StreamError
 from sleekxmpp.stanza.iq import Iq
 from sleekxmpp.stanza.message import Message
 from sleekxmpp.stanza.presence import Presence