s3: Use returned ETag as authoritative MD5 content hash

The Amazon documentation doesn't promise it, but the ETag returned from a PUT request seems to always contain the MD5 hash of the file contents as received by Amazon AWS. Therefore, there is no need to download the entire file contents to check the hash of the contents locally on the web server.

This is not true for multipart uploads, but the plugin doesn't do multipart uploads yet.

References:
http://stackoverflow.com/a/1775831/1025836