Add StateHandler to contrib_bots/run.py.

This allows bots to be stateful.  It doesn't handle persistence
after the bot shuts down, but it does store state between
invocations of handle_message.

contrib_bots/lib/followup.py

@@ -37,7 +37,7 @@ class FollowupHandler(object):
 
         return is_follow_up
 
-    def handle_message(self, message, client):
+    def handle_message(self, message, client, state_handler):
         original_content = message['content']
         original_sender = message['sender_email']
         new_content = original_content.replace('@followup',