Added support for copy on multipart_upload

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
Adjustments as per feedback from AWS developer
forgot to remove debug statement
Merge branch 'master' of https://github.com/highspot/aws-sdk-ruby into highspot-master
Added integration tests for multipart copy.

References #187