Trims a clip to be duration starting at start
        @param video_name : name of the input video
        @param out : name of the output video
        @param start : starting position after the trim
        @param duration : duration of video after start

def trim(self, video_name, out, start, duration):
        """
        Trims a clip to be duration starting at start
        @param video_name : name of the input video
        @param out : name of the output video
        @param start : starting position after the trim
        @param duration : duration of video after start
        """
        command = ['ffmpeg', '-ss', start, '-i', video_name, '-c:v', 'huffyuv',
                   '-y', '-preset', 'veryslow', '-t', duration, out]
        if self.verbose:
            print 'Trimming {0} into {1}'.format(video_name, out)
            print ' '.join(command)
        call(command)