Performance fix

Hi folks,
When i played my game on another computer i realised, that the quick hack i made to run the game at 30 fps may does not work on other computers as well. So i changed it. So if the games running slow (you can indicate this, when the time counter in the upper left decreases the seconds too slow), then please use my fix.

For this replace the function start_game_loop() in lib/ with the following code fragment:
def start_game_loop( self ):
    while self.run_game:
      ticks_start = pygame.time.get_ticks()
      # handle events
      # update objects
      # draw stuff
      # check wining condition
      ticks_end = pygame.time.get_ticks()
      # wait a little to get 30fps
      ticks_diff = ticks_end - ticks_start
      if ticks_diff < 1000/frames_per_second:
        pygame.time.delay( 1000/int(frames_per_second)-ticks_diff )