Update the documentation and examples

- update most of the examples with slixmpp
- change the help channels pointed out in the doc
- add a page listing differences from slixmpp and how to use asyncio
  nicely with slixmpp
- fix some in-code rst documentation

examples/migrate_roster.py

@@ -103,5 +103,5 @@ def on_session2(event):
         new_xmpp.disconnect()
 new_xmpp.add_event_handler('session_start', on_session2)
 
-if new_xmpp.connect():
-    new_xmpp.process(block=True)
+new_xmpp.connect()
+new_xmpp.process()