• Switch the order of "policy" and "known_host_path" arguments
  in method "check_hostkey".
  
• Improve "check_hostkey" method adding support for using a
  callback as policy.
  
• Rename "remote_port" to "port", "remote_hostname" to
  "hostname" and "check_remote_hostkey" to "check_hostkey".
  
• Rewrite constant generation code
  
• Several documentation improvements.