setup configuration structure to hold server secret
use server-wide configuration API for server secret
enable setting the server secret with config directive
rewrite README entry on server secret configuration
Merge pull request #11 from mozilla/secret-option

Secret option
alert on errors, and delay button enabling when verifying assertion
update reqwest library to latest
use const in cookie utils, parameters are read only
remove file guards from verify.c
propogate verification errors up, improve assertion processing API
raise errors to user upon verification failure