ported MailChimp-specific functions from go-mcsync to go-mailchimp

plus some cleanup and removal of unused code

see https://github.com/GigaOM/legacy-pro/issues/3526
Github [claims I authored this file](https://github.com/GigaOM/go-mcsync/commits/master/components/class-go-mcsync-test.php), but I don't remember doing so.

Should we drop this file?