Add fancy locking to oauth2client.

Reviewed in http://codereview.appspot.com/4919049/
diff --git a/oauth2client/django_orm.py b/oauth2client/django_orm.py
index c818ea2..581fe8e 100644
--- a/oauth2client/django_orm.py
+++ b/oauth2client/django_orm.py
@@ -99,7 +99,7 @@
     if len(entities) > 0:
       credential = getattr(entities[0], self.property_name)
       if credential and hasattr(credential, 'set_store'):
-        credential.set_store(self.put)
+        credential.set_store(self)
     return credential
 
   def put(self, credentials):