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/rpc_async.py

@@ -20,7 +20,7 @@ class Boomerang(Endpoint):
 
     @remote
     def throw(self):
-        print "Duck!"
+        print("Duck!")