Add support for client_secrets.json file format.

Reviewed in http://codereview.appspot.com/4956059/.
diff --git a/samples/buzz/buzz.py b/samples/buzz/buzz.py
index 8906048..6334ed6 100644
--- a/samples/buzz/buzz.py
+++ b/samples/buzz/buzz.py
@@ -38,31 +38,44 @@
 import gflags
 import httplib2
 import logging
+import os
 import pprint
 import sys
 
 from apiclient.discovery import build
 from oauth2client.file import Storage
 from oauth2client.client import AccessTokenRefreshError
-from oauth2client.client import OAuth2WebServerFlow
+from oauth2client.client import flow_from_clientsecrets
 from oauth2client.tools import run
 
+
 FLAGS = gflags.FLAGS
 
-# Set up a Flow object to be used if we need to authenticate. This
-# sample uses OAuth 2.0, and we set up the OAuth2WebServerFlow with
-# the information it needs to authenticate. Note that it is called
-# the Web Server Flow, but it can also handle the flow for native
-# applications <http://code.google.com/apis/accounts/docs/OAuth2.html#IA>
-# The client_id client_secret are copied from the API Access tab on
-# the Google APIs Console <http://code.google.com/apis/console>. When
-# creating credentials for this application be sure to choose an Application
-# type of "Installed application".
-FLOW = OAuth2WebServerFlow(
-    client_id='433807057907.apps.googleusercontent.com',
-    client_secret='jigtZpMApkRxncxikFpR+SFg',
+# CLIENT_SECRETS, name of a file containing the OAuth 2.0 information for this
+# application, including client_id and client_secret, which are found
+# on the API Access tab on the Google APIs
+# Console <http://code.google.com/apis/console>
+CLIENT_SECRETS = 'client_secrets.json'
+
+# Helpful message to display in the browser if the CLIENT_SECRETS file
+# is missing.
+MISSING_CLIENT_SECRETS_MESSAGE = """
+WARNING: Please configure OAuth 2.0
+
+To make this sample run you will need to populate the client_secrets.json file
+found at:
+
+   %s
+
+with information from the APIs Console <https://code.google.com/apis/console>.
+
+""" % os.path.join(os.path.dirname(__file__), CLIENT_SECRETS)
+
+# Set up a Flow object to be used if we need to authenticate.
+FLOW = flow_from_clientsecrets(CLIENT_SECRETS,
     scope='https://www.googleapis.com/auth/buzz',
-    user_agent='buzz-cmdline-sample/1.0')
+    message=MISSING_CLIENT_SECRETS_MESSAGE)
+
 
 # The gflags module makes defining command-line options easy for
 # applications. Run this program with the '--help' argument to see