Enable user authentication by access token

As the case written in http://stackoverflow.com/questions/4623974/design-for-facebook-authentication-in-an-ios-app-that-also-accesses-a-secured-we, it is possible that access tokens obtained from mobile apps (iOS or Android) would be used for server side authentication.

This change set retrieves access_token pass by request during facebook login check to enable facebook user authentication by access token. This also handles the issue #210.