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?
True, but since deleting test user can be done with either `app_access_token` or `access_token` of the test user, wouldn't it be ambiguous if it's not explicitly mentioned?