strconv: add example for Unquote.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/38920043

src/pkg/strconv/quote_example_test.go

+// Copyright 2013 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.
+
+package strconv_test
+
+import (
+	"fmt"
+	"strconv"
+)
+
+func ExampleUnquote() {
+	test := func(s string) {
+		t, err := strconv.Unquote(s)
+		if err != nil {
+			fmt.Printf("Unquote(%#v): %v\n", s, err)
+		} else {
+			fmt.Printf("Unquote(%#v) = %v\n", s, t)
+		}
+	}
+
+	s := `cafe\u0301`
+	// If the string doesn't have quotes, it can't be unquoted.
+	test(s) // invalid syntax
+	test("`" + s + "`")
+	test(`"` + s + `"`)
+
+	test(`'\u00e9'`)
+
+	// Output:
+	// Unquote("cafe\\u0301"): invalid syntax
+	// Unquote("`cafe\\u0301`") = cafe\u0301
+	// Unquote("\"cafe\\u0301\"") = café
+	// Unquote("'\\u00e9'") = é
+}