adding APIs for app test user.

This commit is sample implementation of #96.
Since `TestGetAppAccessToken ` from `test/test_facebook.py` does not guarantees a valid app access token, how should I write meaningful test codes?
Okay, that makes sense. It would be good to add a comment explaining *why* we are not using `delete_object` here (because it would prevent us from using the app access token to delete test users), so someone (maybe me...) does not try to "optimize" this in the future.