Update native-messaging to use python3, clarify instructions

Fixes #560

native-messaging/app/ping_pong.json

@@ -1,7 +1,7 @@
 {
   "name": "ping_pong",
   "description": "Example host for native messaging",
-  "path": "/path/to/native-messaging/app/ping_pong.py",
+  "path": "/Users/svincent/dev/webextensions-examples/native-messaging/app/ping_pong.py",
   "type": "stdio",
   "allowed_extensions": [ "ping_pong@example.org" ]
 }

native-messaging/app/ping_pong.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import sys
 import json
@@ -19,7 +19,7 @@ try:
     # given its content.
     def encodeMessage(messageContent):
         # https://docs.python.org/3/library/json.html#basic-usage
-        # To get the most compact JSON representation, you should specify 
+        # To get the most compact JSON representation, you should specify
         # (',', ':') to eliminate whitespace.
         # We want the most compact representation because the browser rejects
         # messages that exceed 1 MB.
@@ -37,6 +37,7 @@ try:
         receivedMessage = getMessage()
         if receivedMessage == "ping":
             sendMessage(encodeMessage("pong3"))
+
 except AttributeError:
     # Python 2.x version (if sys.stdin.buffer is not defined)
     # Read a message from stdin and decode it.
@@ -52,7 +53,7 @@ except AttributeError:
     # given its content.
     def encodeMessage(messageContent):
         # https://docs.python.org/3/library/json.html#basic-usage
-        # To get the most compact JSON representation, you should specify 
+        # To get the most compact JSON representation, you should specify
         # (',', ':') to eliminate whitespace.
         # We want the most compact representation because the browser rejects
         # messages that exceed 1 MB.

native-messaging/app/ping_pong_win.bat

@@ -1,3 +1,3 @@
 @echo off
 
-call python C:\path\to\ping_pong.py
+call python3 C:\path\to\ping_pong.py