Get all files in a folder recursively

It would be great if the SDK offered a way to get all files regardless of "depth". In issue #271, there is a suggestion to use iterators and that in turn results in chained promise requests. I can't imagine this is the optimal way to do such procedure. The SDK client should make one request and the API server should climb down iteratively. Preferably something like this:

```js
client.folders.getItems('0', { recursive: true })
```

The API, SDK, Control Panel and the documentation looks awesome - supreme when compered to MS OneDrive for instance. But they do not support recursive option either while many other cloud storage SDK's do. I hope you will consider adding this feature.

Thanks in advance