Store md5 as metadata instead of a separate object

[taylorskalyo]

Maybe we can add a new flag that, when specified, stores the MD5 value as a metadata field instead of as a separate object.

[taylorskalyo]

After thinking about this some more, it may be more difficult than I initially thought. Because gof3r accepts an input stream, we don't know the MD5 value until the end of the upload. However, in order to put it in the object metadata, we need to know the value at the beginning of the upload. S3 doesn't make it easy to change metadata after upload.