Add support for Google Compute Engine service accounts.

Reviewed in http://codereview.appspot.com/6441118/.
diff --git a/oauth2client/client.py b/oauth2client/client.py
index 6b1bdbd..18a6ce1 100644
--- a/oauth2client/client.py
+++ b/oauth2client/client.py
@@ -624,7 +624,7 @@
           self.invalid = True
           if self.store:
             self.store.locked_put(self)
-      except:
+      except StandardError:
         pass
       raise AccessTokenRefreshError(error_msg)