Use tar checksum instead of md5 of the files and fix memory bloat

[ChrisCinelli]

Fixes #27 and point 1 in #90

[wagoodman]

Thanks for the PR, however, based on the feed back from #90 (comment) I'm going to close this PR.

Also, I was looking for a way to get this functionality without forking the tar lib and it came down to this approach:

	hdrBuf := new(bytes.Buffer)

	tarFile, err := os.Open("image.tar")
	if err != nil {
		fmt.Printf("ERROR: %v\n", err)
	}

	tr := tar.NewReader(io.TeeReader(tarFile, hdrBuf))

	for {
		hdrBuf.Reset()
		header, err := tr.Next()
		if err == io.EOF {
			break
		} 
		hdrBuf.Next(hdrBuf.Len() % 512)

		fmt.Printf("%s\n", hex.Dump(hdrBuf.Bytes()))
		fmt.Printf("Type: %v\n", header.Format)
		fmt.Printf("Chksum: %v\n", string(hdrBuf.Bytes()[148:][:8]))
	}

This way we could grab only the bytes we need and avoid drifting implementation from forking tar.