Fix XEP-0078 using the new stream feature workflow.

Honestly, this is mainly just a demo/proof of concept that we
can handle dependencies and ordering issues with stream features.

DON'T use XEP-0078 if you are able to use the normal SASL method,
which should be the case unless you are dealing with a very old
XMPP server implementation.

sleekxmpp/stanza/stream_features.py

@@ -19,6 +19,8 @@ class StreamFeatures(StanzaBase):
     namespace = 'http://etherx.jabber.org/streams'
     interfaces = set(('features', 'required', 'optional'))
     sub_interfaces = interfaces
+    plugin_tag_map = {}
+    plugin_attrib_map = {}
 
     def setup(self, xml):
         StanzaBase.setup(self, xml)