Scoreboard

This PR introduces a scoreboard subsystem. The implementation is relatively heavy (lots of string manipulation), but I have highlighted several places that can be optimized.

Bonus feature: Team-based friendly fire control.

Statistics and achievements should be implemented before tackling the E_OBJECTIVE_STAT_* objectives.

As for the actual /scoreboard command, is there a special interface that I have to implement, or shall I hardcode it?

Do not merge yet - The (de)serialization logic is missing.