Freebird: the new go-recurly plugin

mostly pulled out of `go-subscriptions` and `go-subscriptions-freebies`.

see https://github.com/GigaOM/legacy-pro/issues/4125
`wp_send_json_success` seems preferable here, but that would change the JSON payload, so probably not worth the refactor.
http://codex.wordpress.org/Function_Reference/wp_send_json_success