FilesAdded Chunking & Exif checking

Hi, I'm a fork newbie, so please forgive me if this is not a standard pull request. 

I needed to make changes so that I could upload > 1000 JPG files starting from a folder drop in Chrome 21+. Currently the jQuery UI spends a lot of time preloading thumbnails when you drop 1000+ files and Chrome often crashed. 

I added 'FilesAdded' chunking and lazyload image preloading so that thumbnails are not created until they are scrolledIntoView in the thumbnail view.

I also added code to check for valid Exif tags either at img preload or resizeImage() just before the UploadFiles POST. This is for my specific requirement to SKIP uploading JPG files that do NOT have a valid Exif tag.

Let me know if any of these changes are of interest to you.