Add Trello integration script

This is a small Python script to create Trello Webhooks for Zulip.

It builds and sends the Trello API requests required to create the
Trello webhook.

zulip/integrations/trello/README.md

@@ -0,0 +1,16 @@
+# Easy Trello integration for Zulip
+
+An easy Trello integration for Zulip
+
+Usage :
+
+1. Fill the needed information in `zulip_trello_config.py` :
+
+    - The bot API KEY,
+    - The Trello API KEY,
+    - The Trello TOKEN,
+    - The Zulip host
+
+2. Execute the script :
+
+    $ python zulip_trello.py <stream_name> <trello_board_name> <trello_board_id>