shogi_server/{game,time_clock}.rb: Adds variations of thinking time calculation:...

shogi_server/{game,time_clock}.rb: Adds variations of thinking time calculation: ChessClock
(current) and StopWatchClock (new).

StopWatchClock, which is usually used at official games of human
professional players, is a clock where thiking time less than a
miniute is regarded as zero.
To select StopWatchClock, use a special game name with "060"
byoyomi time. ex. "gamename_1500_060".