Consider adding support for message authentication

[chrishiestand]

Support could be added either directly or by ensuring it's easy for a third party package to extend webhooks with this functionality.

Some uses of webhooks may allow for multiple senders with different permissions or handle sensitive data that needs to be verified to be both accurate and authenticated.

One solution to this problem is HMAC which uses a shared secret key and is already implemented in the Python Standard Library:
https://docs.python.org/3.4/library/hmac.html

As a point of comparison, you can see how the financial service Dwolla uses HMAC for their webhooks:
https://developers.dwolla.com/dev/pages/webhooks