Proposal implementation of a new command called MONITOR2{ON,OFF}.

Proposal implementation of a new command called MONITOR2{ON,OFF}.

* [shogi-server]
  - shogi_server/command.rb, test/TC_command.rb
    Proposal implementation of a new command called MONITOR2{ON,OFF}.
    When the MONITOR2ON command is issued by a player, the server
    immediately sends the player the entire contents of a record
    file of the game, i.e. history of moves and so on, at that time.
    Then, the server will forward subsequent moves like they are
    appended to the record file.
  - Behavior changed: A player monitoring a game with MONITORON will
    not receive Game#show again when a gaming player resigns (:toryo).