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
Totally agree. see https://github.com/GigaOM/legacy-pro/issues/4192. I think since `go-subscriptions` was using a class var meta_key_prefix, its meta keys should only be accessed via the plugin. but I found at least `go-google-analytics` that references `go-subscriptions`'s user meta name directly. I'll add that to 4192.