Add a Go language example.

This follows the other examples so that it can be used as a tutorial,
such as the ones at:
  https://developers.google.com/protocol-buffers/docs/tutorials

Even though Go generally does not use Makefiles, I added targets for the
Go examples to be consistent with the other languages.

Edit:

Fix Travis run. Change to use $HOME instead of ~. Add protoc to path.
GOPATH entry cannot start with shell metacharacter '~': "~/gocode"

Edit(2):

Fix Go code style to address comments.

1 files changed, 59 insertions, 0 deletions

diff --git a/examples/list_people.go b/examples/list_people.go
new file mode 100644
index 00000000..48b1fbfa
--- /dev/null
+++ b/examples/list_people.go
@@ -0,0 +1,59 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+
+	"github.com/golang/protobuf/proto"
+	pb "github.com/google/protobuf/examples/tutorial"
+)
+
+func listPeople(w io.Writer, book *pb.AddressBook) {
+	for _, p := range book.People {
+		fmt.Fprintln(w, "Person ID:", p.Id)
+		fmt.Fprintln(w, "  Name:", p.Name)
+		if p.Email != "" {
+			fmt.Fprintln(w, "  E-mail address:", p.Email)
+		}
+
+		for _, pn := range p.Phones {
+			switch pn.Type {
+			case pb.Person_MOBILE:
+				fmt.Fprint(w, "  Mobile phone #: ")
+			case pb.Person_HOME:
+				fmt.Fprint(w, "  Home phone #: ")
+			case pb.Person_WORK:
+				fmt.Fprint(w, "  Work phone #: ")
+			}
+			fmt.Fprintln(w, pn.Number)
+		}
+	}
+}
+
+// Main reads the entire address book from a file and prints all the
+// information inside.
+func main() {
+	if len(os.Args) != 2 {
+		log.Fatalf("Usage:  %s ADDRESS_BOOK_FILE\n", os.Args[0])
+	}
+	fname := os.Args[1]
+
+	// Read the existing address book.
+	in, err := ioutil.ReadFile(fname)
+	if err != nil {
+		if os.IsNotExist(err) {
+			fmt.Printf("%s: File not found.  Creating new file.\n", fname)
+		} else {
+			log.Fatalln("Error reading file:", err)
+		}
+	}
+	book := &pb.AddressBook{}
+	if err := proto.Unmarshal(in, book); err != nil {
+		log.Fatalln("Failed to parse address book:", err)
+	}
+
+	listPeople(os.Stdout, book)
+}