Add support for commenting for reply instead of non-threaded messages

Commenting will now be the default action when msg.sending, providing
the needed info has been captured from the stream event.

Script authors can force a new message by modifying the response
envelope: msg.envelope.newMessage = true; msg.send …

This requires the new comment method in node-flowdock 0.6

Fixes #39
Merge pull request #40 from brentc/commenting

Add support for commenting for reply instead of non-threaded messages