lose accuracy with using float for timetick

in the code 

```
float HelloWorld::getTimeTick() {
    timeval time;
    gettimeofday(&time, NULL);
    unsigned long millisecs = (time.tv_sec * 1000) + (time.tv_usec / 1000);
    //the millisecs is always 13 demical digits long, however float only have at most 9 significant demical digits.
    return (float)millisecs;
}
```

As a result, there is only one asteroid spawned in a game.
