Support http-01 challenge

[jlindsey]

Utilize the web server to respond to http-01 challenges.

High-level overview:

• Update configs to be more general (ie. get rid of dns_config in favor of something like auth_config or acme_auths)
• Update SiteConfigInternal to reference new authorization methods
• Update CreateACMECert() to handle this type of authorization
• Update web server to respond properly to the challenge