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
Elsewhere I suggested moving the hook that calls this to the main class file. I mean that, but I'd leave this right here. In the main class file I'd make a method that then sub-calls `freebie()->signup( ... )`.