Add a command sender.

[notaphplover]

Important: This feature requires #66, #68 to be closed before starting any development process related to this issue.

The game needs a tool to send commands to the players.

• A command sender interface must be added in order to send commands.
• Game engine must be able to contain a command sender.
• Game engine handlers must be updated in order to send commands:
  • An end turn request handler must send a command to start playing the next turn. If the game is ended, it must send a command to stop players.
  • A start turn request must send a command to play the turn with the game state.
  • Any other request must send a continue playing turn command (as a kind of ACK).