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 
I think that the word renew is potentially problematic here.  Consider using the word "fetch" or something similar.  Then - subsequently - explain that the CA is able to update the certificate at its discretion, allowing for shorter validity windows.