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
Pig-piling or dog-piling, or something, on @borkweb:

![screen shot 2014-09-23 at 7 25 24 am](https://cloud.githubusercontent.com/assets/1709697/4373552/7e9b99d2-432d-11e4-99fa-c3287c190803.png)