Implement first client command

The client can now talk to the old Perl signer implementation. Running
socat has been documented in README.md. Commonly used I/O code has been
moved to the shared/io.go file.

shared/io.go

@@ -0,0 +1,31 @@
+package shared
+
+import (
+	"errors"
+	"github.com/goburrow/serial"
+	"io"
+	"time"
+)
+
+// receive the requested number of bytes from serial port and stop after the given timeout in seconds
+func ReceiveBytes(port *serial.Port, count int, timeout time.Duration) ([]byte, error) {
+	timeoutCh := time.After(timeout * time.Second)
+	readCh := make(chan []byte, 1)
+	errCh := make(chan error, 1)
+	go func() {
+		data := make([]byte, count)
+		if _, err := io.ReadAtLeast(*port, data, count); err != nil {
+			errCh <- err
+		} else {
+			readCh <- data
+		}
+	}()
+	select {
+	case <-timeoutCh:
+		return nil, errors.New("timeout")
+	case err := <-errCh:
+		return nil, err
+	case data := <-readCh:
+		return data, nil
+	}
+}

shared/shared.go

@@ -1,4 +1,3 @@
 package shared
 
 const MagicTrailer = "rie4Ech7"
-