Add a Tasks sample that demonstrates Service accounts.

Reviewed in http://codereview.appspot.com/5685068/.

Index: samples/service_account/books.py
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diff --git a/samples/service_account/tasks.py b/samples/service_account/tasks.py
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+++ b/samples/service_account/tasks.py
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+#!/usr/bin/python2.4
+# -*- coding: utf-8 -*-
+#
+# Copyright (C) 2012 Google Inc.
+#
+# 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
+#
+#      http://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.
+
+"""Simple command-line sample that demonstrates service accounts.
+
+Lists all the Google Task Lists associated with the given service account.
+Service accounts are created in the Google API Console. See the documentation
+for more information:
+
+   https://developers.google.com/console/help/#WhatIsKey
+
+Usage:
+  $ python tasks.py
+"""
+
+__author__ = 'jcgregorio@google.com (Joe Gregorio)'
+
+import httplib2
+import pprint
+import sys
+
+from apiclient.discovery import build
+from oauth2client.client import SignedJwtAssertionCredentials
+
+def main(argv):
+  # Load the key in PKCS 12 format that you downloaded from the Google API
+  # Console when you created your Service account.
+  f = file('key.p12', 'rb')
+  key = f.read()
+  f.close()
+
+  # Create an httplib2.Http object to handle our HTTP requests and authorize it
+  # with the Credentials. Note that the first parameter, service_account_name,
+  # is the Email address created for the Service account. It must be the email
+  # address associated with the key that was created.
+  credentials = SignedJwtAssertionCredentials(
+      '141491975384@developer.gserviceaccount.com',
+      key,
+      scope='https://www.googleapis.com/auth/tasks')
+  http = httplib2.Http()
+  http = credentials.authorize(http)
+
+  service = build("tasks", "v1", http=http)
+
+  # List all the tasklists for the account.
+  lists = service.tasklists().list().execute(http)
+  pprint.pprint(lists)
+
+
+if __name__ == '__main__':
+  main(sys.argv)