Make authn sample more uniform with the java one.

bigquery/samples/utils.py

@@ -21,9 +21,16 @@ def get_service():
     from oauth2client.client import GoogleCredentials
 
     credentials = GoogleCredentials.get_application_default()
+
+    # Some credential types require you to specify scopes, and service entry
+    # points may not inject them. Check for this case, and inject the scopes
+    # explicitly if required.
     if credentials.create_scoped_required():
         credentials = credentials.create_scoped(
             'https://www.googleapis.com/auth/bigquery')
+
+    # Otherwise, the build() method takes care of injecting the proper scopes
+    # for the given service.
     return build('bigquery', 'v2', credentials=credentials)
 # [END get_service]