RESTification

This PR closes #23.  It updates the overall flow of the protocol to make better use of HTTP.  There are no changes to the validation challenges.  In case it helps, there's a side-by-side comparison of protocol messages here:

http://www.ipv.sx/tmp/restification.html 
Having done the URN thing recently, you can copy it.  Here: http://martinthomson.github.io/drafts/draft-thomson-webpush-http2.html#rfc.section.9 and here https://github.com/martinthomson/drafts/blob/master/push-http2/draft-thomson-webpush-http2.xml