Add SMS support

Summary:
Provides a working SMS implementation with support for Twilio.

This version doesn't really retry if we get any gruff at all. Future versions should retry.

Test Plan: used bin/sms to send messages and look at them.

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: aurelijus, epriestley, Korvin

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D8930

externals/twilio-php/Services/Twilio/Rest/ShortCode.php

@@ -0,0 +1,6 @@
+<?php
+
+class Services_Twilio_Rest_ShortCode
+    extends Services_Twilio_InstanceResource
+{
+}