more debug output plus a couple of cleanup fixes

* removed an unused object var $debug
* more slogging when `go-syncuser()->debug()` is `TRUE`
* removed some extraneous calls to [`subscribe()`](https://github.com/GigaOM/go-mailchimp/blob/4edb240d8584a5c6d6e9a2d36436d1284b0ff12f/components/class-go-mailchimp-api.php#L331) and [`unsubscribe()`](https://github.com/GigaOM/go-mailchimp/blob/4edb240d8584a5c6d6e9a2d36436d1284b0ff12f/components/class-go-mailchimp-api.php#L481) due to mismatched array format passed in by [`user_sync_ajax()`](https://github.com/GigaOM/go-mailchimp/blob/4edb240d8584a5c6d6e9a2d36436d1284b0ff12f/components/class-go-mailchimp-admin.php#L184)
* added a nonce to the "Sync to MailChimp" button
* use `wp_die()` instead of `die`

depends on [this pull request](https://github.com/GigaOM/go-syncuser/pull/5)

see https://github.com/GigaOM/legacy-pro/issues/4020
isn't the debug flag being persisted in a site_option though?  So not setting it back may leave it on for future requests?