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?
I think it might be simpler and more consistent if all methods were called `*_test_user` (i.e. `create_test_user`, `edit_test_user`, `get_test_users`, etc.).