[CHEF-5356] Encrypted data bags should use different HMAC key and include the IV in the HMAC

https://tickets.opscode.com/browse/CHEF-5356
By the way, encoding the secret in base64 like in the official documentation makes me a little nervous. But an [internal `#strip`](http://git.io/5B8IXA) do not let us use binary secrets  :confused: (ok, subject for another ticket).