Optionally run uploads in the background

[lenkawell]

Since it can take quite a few minutes to upload a lot of photos or a photo book, it would be really great if the uploads occurred completely in the background. The NSURLSessionUploadTask service might be a way to handle this (I see that you're already using the NSURLSessionDownloadTask).

The app could then use the new print order history API to provide the user with the current status of the background upload.