Get a search query from the caller

The caller enters up to three digits to select who they want to be
connected to, and Twilio will POST that information back to us.

For testing purposes, for now we just read the input back to the caller.
Pretty up Twilio response syntax

The `twilio-ruby` gem [1] gives us a nice Builder::XmlMarkup derived
syntax for generating TwiML, without writing out ugly XML heredocs.