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 
Nice.  At least this way the cert can sit in an HTTP cache.  Noting of course the limit on expiration that HTTP advises, which is much lower than the typical certificate lifetime.