Add Collector stanza handler class.

This style of handler is necessary for capturing result sets from
queries that use multiple messages to send the results instead of
in a single result stanza. Notably, XEP-0313 (MAM).

sleekxmpp/xmlstream/handler/__init__.py

@@ -7,6 +7,7 @@
 """
 
 from sleekxmpp.xmlstream.handler.callback import Callback
+from sleekxmpp.xmlstream.handler.collector import Collector
 from sleekxmpp.xmlstream.handler.waiter import Waiter
 from sleekxmpp.xmlstream.handler.xmlcallback import XMLCallback
 from sleekxmpp.xmlstream.handler.xmlwaiter import XMLWaiter