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
Confirmed. `class-go-recurly-freebies.php` really should _not_ be instantiated all the time. For the cost of adding the hook and another method in this class we can avoid loading it on the vast majority of page loads. See https://github.com/GigaOM/go-recurly/pull/5#discussion_r17636310