Move Go code to src/cmd

This does not get everything where we want it finally, see #1266
nor the optimal way of building, but it gets it out of top level.

Added instructions to build if you have a Go installation.

Not moving `vendor` yet.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

src/pad4/pad4.go

@@ -0,0 +1,46 @@
+package pad4
+
+import (
+	"bytes"
+	"io"
+)
+
+// A Writer is an io.WriteCloser. Writes are padded with zeros to 4 byte boundary
+type Writer struct {
+	w     io.Writer
+	count int
+}
+
+// Write writes output
+func (pad Writer) Write(p []byte) (int, error) {
+	n, err := pad.w.Write(p)
+	if err != nil {
+		return 0, err
+	}
+	pad.count += n
+	return n, nil
+}
+
+// Close adds the padding
+func (pad Writer) Close() error {
+	mod4 := pad.count & 3
+	if mod4 == 0 {
+		return nil
+	}
+	zero := make([]byte, 4-mod4)
+	buf := bytes.NewBuffer(zero)
+	n, err := io.Copy(pad.w, buf)
+	if err != nil {
+		return err
+	}
+	pad.count += int(n)
+	return nil
+}
+
+// NewWriter provides a new io.WriteCloser that zero pads the
+// output to a multiple of four bytes
+func NewWriter(w io.Writer) *Writer {
+	pad := new(Writer)
+	pad.w = w
+	return pad
+}