Add fancy locking to oauth2client.

Reviewed in http://codereview.appspot.com/4919049/
diff --git a/oauth2client/appengine.py b/oauth2client/appengine.py
index 439a579..64fd3ac 100644
--- a/oauth2client/appengine.py
+++ b/oauth2client/appengine.py
@@ -23,7 +23,6 @@
 import pickle
 import time
 import base64
-import logging
 
 try: # pragma: no cover
   import simplejson
@@ -222,7 +221,7 @@
     entity = self._model.get_or_insert(self._key_name)
     credential = getattr(entity, self._property_name)
     if credential and hasattr(credential, 'set_store'):
-      credential.set_store(self.put)
+      credential.set_store(self)
       if self._cache:
         self._cache.set(self._key_name, pickle.dumps(credentials))