Improve listening usability

Add Listener.register and Listener.run

examples/echobot.py

@@ -1,17 +1,15 @@
 import fbchat
 
 session = fbchat.Session.login("<email>", "<password>")
-
-listener = fbchat.Listener.connect(session, chat_on=False, foreground=False)
+listener = fbchat.Listener(session=session, chat_on=False, foreground=False)
 
 
-def on_message(event):
+@listener.register
+def on_message(event: fbchat.MessageEvent):
     print(f"{event.message.text} from {event.author.id} in {event.thread.id}")
     # If you're not the author, echo
     if event.author.id != session.user.id:
         event.thread.send_text(event.message.text)
 
 
-for event in listener.listen():
-    if isinstance(event, fbchat.MessageEvent):
-        on_message(event)
+listener.run()