add gvisor based service library (#965)

* add service/ library

examples/go_service/main.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+
+	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/service"
+)
+
+func main() {
+	if err := run(); err != nil {
+		log.Fatalf("%+v", err)
+	}
+}
+
+func run() error {
+	configStr := `
+tun:
+  user: true
+
+static_host_map:
+  '192.168.100.1': ['localhost:4242']
+
+listen:
+  host: 0.0.0.0
+  port: 4241
+
+lighthouse:
+  am_lighthouse: false
+  interval: 60
+  hosts:
+    - '192.168.100.1'
+
+firewall:
+  outbound:
+    # Allow all outbound traffic from this node
+    - port: any
+      proto: any
+      host: any
+
+  inbound:
+    # Allow icmp between any nebula hosts
+    - port: any
+      proto: icmp
+      host: any
+    - port: any
+      proto: any
+      host: any
+
+pki:
+  ca: /home/rice/Developer/nebula-config/ca.crt
+  cert: /home/rice/Developer/nebula-config/app.crt
+  key: /home/rice/Developer/nebula-config/app.key
+`
+	var config config.C
+	if err := config.LoadString(configStr); err != nil {
+		return err
+	}
+	service, err := service.New(&config)
+	if err != nil {
+		return err
+	}
+
+	ln, err := service.Listen("tcp", ":1234")
+	if err != nil {
+		return err
+	}
+	for {
+		conn, err := ln.Accept()
+		if err != nil {
+			log.Printf("accept error: %s", err)
+			break
+		}
+		defer conn.Close()
+
+		log.Printf("got connection")
+
+		conn.Write([]byte("hello world\n"))
+
+		scanner := bufio.NewScanner(conn)
+		for scanner.Scan() {
+			message := scanner.Text()
+			fmt.Fprintf(conn, "echo: %q\n", message)
+			log.Printf("got message %q", message)
+		}
+
+		if err := scanner.Err(); err != nil {
+			log.Printf("scanner error: %s", err)
+			break
+		}
+	}
+
+	service.Close()
+	if err := service.Wait(); err != nil {
+		return err
+	}
+	return nil
+}