Copy over Wire tutorial from Go Cloud (#86)

Originally authored by @enocom. Added to CONTRIBUTORS.

Fixes #80

_tutorial/main.go

@@ -0,0 +1,83 @@
+// Copyright 2018 The Wire Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// The greeter binary simulates an event with greeters greeting guests.
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"time"
+)
+
+// Message is what greeters will use to greet guests.
+type Message string
+
+// NewMessage creates a default Message.
+func NewMessage(phrase string) Message {
+	return Message(phrase)
+}
+
+// NewGreeter initializes a Greeter. If the current epoch time is an even
+// number, NewGreeter will create a grumpy Greeter.
+func NewGreeter(m Message) Greeter {
+	var grumpy bool
+	if time.Now().Unix()%2 == 0 {
+		grumpy = true
+	}
+	return Greeter{Message: m, Grumpy: grumpy}
+}
+
+// Greeter is the type charged with greeting guests.
+type Greeter struct {
+	Grumpy  bool
+	Message Message
+}
+
+// Greet produces a greeting for guests.
+func (g Greeter) Greet() Message {
+	if g.Grumpy {
+		return Message("Go away!")
+	}
+	return g.Message
+}
+
+// NewEvent creates an event with the specified greeter.
+func NewEvent(g Greeter) (Event, error) {
+	if g.Grumpy {
+		return Event{}, errors.New("could not create event: event greeter is grumpy")
+	}
+	return Event{Greeter: g}, nil
+}
+
+// Event is a gathering with greeters.
+type Event struct {
+	Greeter Greeter
+}
+
+// Start ensures the event starts with greeting all guests.
+func (e Event) Start() {
+	msg := e.Greeter.Greet()
+	fmt.Println(msg)
+}
+
+func main() {
+	e, err := InitializeEvent("hi there!")
+	if err != nil {
+		fmt.Printf("failed to create event: %s\n", err)
+		os.Exit(2)
+	}
+	e.Start()
+}