SwiftyDropbox: uploading from an asset file

Great next step, thanks! But. (isn't there always?). A [more] common use case for file uploading is asset uploading (image/videos etc). (scheme: assets-library) which won't work with this new code. Yes, you could pick up an 'assets-library to NSInputStream library' (wheee, more code!), but it is simple enough to add it by reading the asset file directly and appending the data as we go.

It would be great to add the capability to the api for directly uploading ALAssets/PHAssets into Dropbox.