Update guide to 1.1, use directory & headers for URIs + other enhance…

…ments

* Use the directory and response headers, rather than hardcoding URIs (closes #1)
* Add Appendix 6 about newly supported Internationalized Domain Names
* Change reference to official Let's Encrypt client → certbot
* Specify a TTL for DNS challenge record
* Add note about certificate and authorization validity periods
* Consistently prefer single quotes in all Ruby code
* Remove example domains for the various certificate types
* Added a couple more tools to the Testing section
* Add Changelog & Author section
* Harden example nginx config with additional security headers

Gemfile

@@ -0,0 +1,7 @@
+source 'https://rubygems.org'
+ruby '2.3.1'
+
+gem 'httparty', '~> 0.14'
+gem 'nitlink', '~> 1.0'
+gem 'dnsimple', '~> 2.2'
+gem 'net-scp', '~> 1.2.1'