• Updating the documentation by pulling the wiki into README.md and making the necessary
  additions and edits.
• If a MDomain is removed, all its challenge store information is purged as well. Test
  case for that added. Fixes #93
• The ACMEv2 endpoint of Let's Encrypt is now the default for new MDs. Existing MDs will keep
  their values unless one explicitly configures a 'MDCertificateAuthority'.
• Non-HTML format of mod_status now lists number of ok/renew/error/ready MDomains.