-Added method countdownTimer() that countdowns from the current time to an selected time
-Method is called in run (in Race) before the simulatedRace
-Private integer PRERACE_TIME is used to set time before the race begins
#story [16]
-Added label to fxml racepane.fxml
-Added calcTime method to calculate time elapsed
-Added updateTime method in Race to update label text
-Added setTimer in RaceController to update label text
#story [16] #pair [fwy13, zwu18]
David Wu