[Reviewer: Mike] ISC headers

Hi Mike - please could you review. This adds the various headers we should be setting and obeying on the ISC interface. I think this now brings that part of our code in line with our spec. I've also removed some demo AS workarounds.

One question - is it better to handle parsing and creating complex headers (like P-Served-User (RFC5502) which has a URI and parameters) in Sprout, or should I be enhancing PJSIP with support for the new headers and just using them in Sprout?

Cheers,

--KW 8-)