Added an init_app method to the Database model.  Also changed the corresponding tests.

The Database constructor's signature has changed from __init__(app, ..) to
__init__(app=None, ..)
Added an init_app method to the Admin model. Also changed the corresponding tests.

The Admin constructor's signature needed to be changed from __init__(app,
auth, ..) to __init__(auth, app=None, ..)
fix: self.app would always be set to None because self.init_app doesn't return anything.