Support idempotency for safely retrying requests without accidentally performing the same operation twice

All requests should be [idempotent](https://en.wikipedia.org/wiki/Idempotence), to ensure that failed requests can be retried without accidentally repeating the same request.

This can be achieved by adding an `idempotency_key` to each requests and the API requestor ensuring that two requests with the same ID are never in flight at the same time.

If the user of the API client fires multiple requests of the same nature they will be assigned a separate `idempotency_key` and a duplicate request will be sent.

The intent of this feature is to ensure that the API client is behaving as expected and duplicate requests are intentional on an application level.

See also #18