Adds multipart copy support

[kerryjj]

This PR sends the required upload_id and part_number to upload_part_copy and implements the parsing of the responses required to make multipart copy work.

See the spec here https://docs.aws.amazon.com/AmazonS3/latest/API/mpUploadUploadPartCopy.html

[kerryjj]

Not sure why I didn't submit this PR at the time I created this.

This fixes the issue mentioned in #46 where the upload_id and part_number are required by S3 and when calling AWS.S3.upload_part_copy.

We also weren't parsing the responses for upload part copy, so I've done that and the feature now works.

[kerryjj]

This PR also fixes #75 because ExAws.S3.Parsers.parse_complete_multipart_upload is now doing what was done in ExAws.S3.Parsers.parse_upload and from what I can see ExAws.S3.Parsers.parse_upload isn't used and can be deleted.

[bernardd]

Hi @kerryjj - thanks very much for the patch (and especially for including tests - those always make me feel more confident about changes), and my apologies for not getting to it sooner. Would you be kind enough to resolve the conflicts with the current master, and then I'll get this merged? Thanks heaps.

[bernardd]

Superseded by #217