Implemented a simple UserIdSource, that stores the user in a cookie

This concept was borrowed from the official example "Spring Social Canvas".

The idea to store the internal user-id in a cookie and later load the data
of the user according to the cookie is inherent insecure and must not be
used in a production environment.

One simply can use Spring-Security instead - we will show how to switch in
a later example.

This implementation was choosen only for educational purposes, because it
clarifys the design of Spring Social.