Feature/editor 207 media upload limit issues

Add a WPMediaProcessor to process and handle media exporting from the library before its uploaded. This sort outs the memory issues we where having when selecting multiple images.

Before the process was all done in the main thread and all images at the same time that made it crash because of lack of memory and also made the main thread hang.

At the moment the limit is increased to 10 but can it can be removed to allows the user any number of uploads.

cc/ @bummytime 
`processAsset` sounds a bit generic in this case... I think using `saveAsset` instead is more descriptive of what this method does.