Add support for verifying Twilio request signatures.

This adds support to the client for verifying the Twilio signature on HTTP requests.  It is based on the documentation [0] and the implementation in the official Python bindings [1].  I notice that this duplicates work done in the as-yet-unmerged #3.

If the shape of this looks good, I can add documentation for it.

[0] https://www.twilio.com/docs/security#validating-requests
[1] https://github.com/twilio/twilio-python