validate requests are from twilio

[YayC]

It might make sense to implement a helper function to validate requests are actually from Twilio by inspecting the X-Twilio-Signature header, full request url, and user's TWILIO_AUTH_TOKEN (as described in their security docs here).

Thanks for sharing this project!

[danielberkompas]

@YayConnolly Good point! I'll look into this.