Add chunk-upload retry

[oliverroick]

Implements re-trying for failing chunk uploads.

I took the AWS example and implemented it the following way

• sendNext starts a new chunk upload. It pops on element form the parts array and starts the upload by calling sendChunk.
• sendChunk initialises the XHR object and returns a promise that resolves if the upload succeeds and rejects when not.
• If the upload fails, we push the failed a request back into parts and call sendNext after a short timeout.
• If no more parts are available and there are no ongoing uploads, we call complete to wrap up the upload.