Initial commit after merging my earlier attempt built on top of
bare node.js. Restarting with express.js and angular.js thanks to
an excellent seed from Brian Ford.

Added routes for the Answer, Hangup, and Message URLs required by
Plivo. Created placeholder pages for GET requests, and for the
Hangup URL, which I haven't explored yet (but is sure to come in
handy). Created a function that sends a simple string of Plivo XML
in answer to a POST request on the answer URL (/in), and a much more
interesting function that uses the Plivo javascript library to
forward all SMS messages received at the Plivo DID to any other
arbitrary SMS-enabled DID. It reads the POST request, and pulls
out the content of the message and the ID of the sender, and
then composes a new message and (currently, for my testing) sends
it to my cellphone, helpfully formatted.

This marks the end of the proof of concept phase.

Features, prettifying, and refinement forthcoming.