Introduce network namespace, update .gitignore and Doxyfile
Update .gitignore with some files and directories
Implement first version of spectator mechanism.

Some remarks:
- Win screen at the end of the five battles is not yet displayed
- Rage quitting during the game is not yet detected as a trigger for resetting the game ;)
- Also fixed that the level is now reset after the "limbo" moment where there is no game
Merge pull request #58 from rayburgemeestre/network_multiplayer

Network multiplayer