bots: Add incrementor bot.

This bot provides a sample implementation
for updating existing messages sent by the
bot.

contrib_bots/bots/incrementor/incrementor.py

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+# See readme.md for instructions on running this code.
+
+
+class IncrementorHandler(object):
+
+    def __init__(self):
+        self.number = 0
+        self.message_id = None
+
+    def usage(self):
+        return '''
+        This is a boilerplate bot that makes use of the
+        update_message function. For the first @-mention, it initially
+        replies with one message containing a `1`. Every time the bot
+        is @-mentioned, this number will be incremented in the same message.
+        '''
+
+    def handle_message(self, message, client, state_handler):
+        self.number += 1
+        if self.message_id is None:
+            result = client.send_reply(message, str(self.number))
+            self.message_id = result['id']
+        else:
+            client.update_message(dict(
+                message_id=self.message_id,
+                content=str(self.number),
+            ))
+
+
+handler_class = IncrementorHandler

contrib_bots/bots/incrementor/readme.md

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+# Incrementor bot
+
+This is a boilerplate bot that makes use of the
+update_message function. For the first @-mention, it initially
+replies with one message containing a `1`. Every time the bot
+is @-mentioned, this number will be incremented in the same message.