Bug fixes + code organization

[Bimde]

Fixed issue where server remains in memory after game ends

• Now the server is removed from the main server list
• The message timer stops
• Server-related threads all close

  Fixed bug where server ends game before finishing sending all outstanding messages

• Now the server prevents new messages from being added to the message queue after the game ends
• Guarantees all messages in queue are sent before ending game by preventing timer being stopped until the size of the message queue is 0

  Added debugging message flag

• Use constant 'DEBUG' in server class to enable / disable debugging messages
• All 'System.out.println' calls transferred to Server#println calls, which allow for significantly less 'if(DEBUG)' statements

  Organized code

• Removed Spectator class as it wasn't being used
• Improved documentation
• Made CentralServer class instantiable and removed static methods / variables
• Moved server assignment from Client to CentralServer
• Removed unused methods

[Bimde]

Hopefully after a bit more testing, we could push this to stable as well.