Update models

Rename user_id to uid for consistency and add validations and
dependencies.
Comment out callback actions

Delegate the callback actions to the client since this is how it would
work using the Facebook mobile SDK.
Change test program to use local server
Implement client-server authentication flow

To get the app token to access the API, the client must first send the
bare minimum of a Facebook token and uid to /authenticate. A successful
authentication will return the app token which must be passed to the
HTTP header Authorization: Token token="...". A newly registered user
will return HTTP code 201, and a returning user will return 200.