Support alternative challenge methods

[jalada]

There's lots of discussion about supporting DNS validations instead of HTTP validations, because it comes with the added benefit of working with wildcard domains.

However, DNS validations require programmatic DNS updates (e.g. via an API), and support for that is sporadic. Last time we checked there's no unified library for handling DNS updates, but if/when that changes we'd consider implementing this. We're not going to add individual API support (e.g. Cloudflare API, Route53 API) to letsencrypt-rails-heroku, that road leads to high-maintenance madness.

Related issues, PRs, and discussions: #59, #60, #62, #69