Turn server backend into a library

This way, the server can be built into zordzman-plus-plus,
providing some COOL FEATURES JUST LIKE MINECRAFT:

- Unified code for single and multiplayer.
  The single player game would just host a server on localhost

- Allow players to host right from the game instead of having
  to fuck with the command line

OF COURSE THE STANDALONE SERVER WILL STILL EXIST, IT WILL USE
THE LIBRARY. HURR DURR?