us020: added close method

samples/us020.go

@@ -9,6 +9,7 @@ import (
 
 func main() {
 	rangeFinder := us020.New(10, 9)
+	defer rangeFinder.Close()
 
 	for {
 		distance, err := rangeFinder.Distance()

sensor/us020/us020.go

@@ -18,6 +18,8 @@ const (
 type US020 interface {
 	// Distance computes the distance of the bot from the closest obstruction.
 	Distance() (float64, error)
+
+	Close()
 }
 
 type us020 struct {
@@ -117,3 +119,8 @@ func (d *us020) Distance() (distance float64, err error) {
 
 	return
 }
+
+func (d *us020) Close() {
+	d.echoPin.Output()
+	rpio.Close()
+}