integrations/trello: Improve the Trello script.

This commit makes a bunch of improvements:

1. Use format() for string formatting instead of the old style.
2. Stop returning stuff where we don't have to.
3. URL generation should not be done by the script, especially
   since our eventual goal is for the Zulip server to automatically
   generate a webhook URL. So the user should be able to supply
   the URL in the config file. Plus, this makes it easier to test
   the script with non-Zulip URLs (such as RequestBin).
4. Removed unnecessary arguments.

zulip/integrations/trello/README.md

@@ -1,16 +1,13 @@
-# Easy Trello integration for Zulip
-
-An easy Trello integration for Zulip
+# A script that automates setting up a webhook with Trello
 
 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
+    - The Zulip webhook URL
 
 2. Execute the script :
 
-    $ python zulip_trello.py <stream_name> <trello_board_name> <trello_board_id>
+    $ python zulip_trello.py <trello_board_name> <trello_board_id>