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].

[0] https://www.twilio.com/docs/security#validating-requests
[1] https://github.com/twilio/twilio-python
Merge pull request #7 from metcalf/am-verify-signature

Add support for verifying Twilio request signatures.