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

docs/api/xmlstream/handler.rst

@@ -10,15 +10,19 @@ The Basic Handler
 
 Callback
 --------
-.. module:: slixmpp.xmlstream.handler.callback
+.. module:: slixmpp.xmlstream.handler
 
 .. autoclass:: Callback
     :members:
 
+CoroutineCallback
+-----------------
+
+.. autoclass:: CoroutineCallback
+    :members:
 
 Waiter
 ------
-.. module:: slixmpp.xmlstream.handler.waiter
 
 .. autoclass:: Waiter
     :members: