Refactored UUID into SessionID interface

Hi,

Introducing AuthorizationHandler was a great step forward, finally we can integrate our websites with netty-socketio. Unfortunately, the session ids used by apps does not have to comply with UUID format.

To overcome this I have created a SessionID interface and an abstract factory SessionIDFactory, with default implementation based on UUID. The users can now created their own implementations of session ids using their formats.

The project is missing unit tests, and I did not have time yet to verify the solution is not breaking any other functionality.

Feel free to merge these changes if you find it useful.