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

plus some cleanup and removal of unused code

related to these two other pull requests:
* https://github.com/GigaOM/go-syncuser/pull/2
* https://github.com/GigaOM/gigaom/pull/4640

see https://github.com/GigaOM/legacy-pro/issues/3526
Add `'use strict';` (see [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode))