Fix for issue #80

fixes #80
------------

This fix makes all the downloads happen to temporary files of type .s3mcd.XXXXX.tmp in the same folder as the target file's. Once the download is complete, the file is renamed to the actual destination. This renaming is atomic in nature; hence any parallel thread or process could work on fully downloaded data (by filtering all files matching .s3cmd.XXXXX.tmp pattern while walking the data directory).