go-tour: add solution to the stringers exercise

Fixes issue 187

LGTM=minux
R=adg, minux
CC=golang-codereviews
https://codereview.appspot.com/191520043

solutions/stringers.go

@@ -0,0 +1,25 @@
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+package main
+
+import "fmt"
+
+type IPAddr [4]byte
+
+func (ip IPAddr) String() string {
+	return fmt.Sprintf("%d.%d.%d.%d", ip[0], ip[1], ip[2], ip[3])
+}
+
+func main() {
+	addrs := map[string]IPAddr{
+		"loopback":  {127, 0, 0, 1},
+		"googleDNS": {8, 8, 8, 8},
+	}
+	for n, a := range addrs {
+		fmt.Printf("%v: %v\n", n, a)
+	}
+}