Render upload progress/completion public

When using some JavaScript-based client-side uploader instead of locally calling upon the `upload()` function, still some tasks need to be done server-side: checking and completing the upload. 

Therefore, I factored out these two tasks from the `upload()` function into public methods `uploadedBytes()` and `completeUpload()`.

I hope these fit the naming conventions and general API philosophy. If not, please let me know.