Add way to know a message is from FB and respond with special characters and additional text

[brendanbabb]

Facebook unblocked the FB Messenger bot and now requests come in through the twilio FB channel to the Heroku instance. Can we make it so we know a request is from FB for logging and returning special characters in the message. Twilio handles the special characters for SMS. https://www.facebook.com/LandAcknowledgement