Phase 1 complete.
SSL didn't work, so skipping Twilio security requirement (note: Twilio
doesn't work with self-signed certs anyway). Other parts of phase 1
completed.
For reference: plan was to define a new Directive for Spray, to
calculate the signature and match it against the supplied signature, and
'reject' the path if either the signature wasn't supplied or there was a
mismatch.