Don't pass empty voice callback parameters.

Currently when optional `CallbackParameters` are blank, they are passed
to the Twilio API as an empty string.  Twilio interprets these as invalid
parameters (e.g. the empty string is an invalid HTTP method).  This change
only sets non-empty parameters on the form.
Merge pull request #6 from metcalf/am-optional-voice-params

Don't pass empty voice callback parameters.
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.