Adding a .revoke() to Credentials. Closes issue 98.

Reviewed in https://codereview.appspot.com/7033052/
diff --git a/tests/test_oauth2client_file.py b/tests/test_oauth2client_file.py
index 07e7608..c954d5e 100644
--- a/tests/test_oauth2client_file.py
+++ b/tests/test_oauth2client_file.py
@@ -32,6 +32,7 @@
 import unittest
 
 from apiclient.http import HttpMockSequence
+from oauth2client import GOOGLE_TOKEN_URI
 from oauth2client import file
 from oauth2client import locked_file
 from oauth2client import multistore_file