Added option to group files in one XHR request based on their file size and the upload file size limit

If your upload limit is say 1MB and you have 100 files of 20Kb you could set the limitMultiFileUploads to 50 to minimize requests. However is there is one 30kb file the batch containing that group will fail. So I coded an uploadSizeLimitMultiFileUploads option that will calculate the max number of files that fit in one request based on a set upload size. I added a 512 bytes overhead for each file for the calculations to be on the safe side. Hope you find these useful, it is very useful if you want to upload 1000s of small files. Thanks

Signed-off-by: Maarten Ottens <m@maartenottens.com>