Add ability to set arbitrary metadata on pushed S3 object

Adding ability to add `x-amz-meta-*` headers [here](https://github.com/backup/backup/blob/master/lib/backup/cloud_io/s3.rb#L205-L219) would seemingly allow us to upload custom metadata fields with our backup, like so:
https://stackoverflow.com/questions/12290819/how-to-upload-s3-metadata-with-a-file-in-fog

This would allow users to get around the fact that S3 doesn't yet allow a means to check the MD5 of the full file when it has been uploaded via multipart. It would allow us to set out own custom metadata with the md5 hash, that we could use ensure S3 files aren't redownloaded if unchanged from expected hash.