Add support for copy on multipart_upload with auto-detection on s3_object copy methods

as per
http://docs.aws.amazon.com/AmazonS3/latest/API/mpUploadUploadPartCopy.ht
ml, this adds an explicit api on the multipart_upload object
(copy_part) as well as auto-detection and handling of file > 5GB for
copy method on an s3_object