Add Heavy Tank

[Sesamexx]

Heavy Tank is based on Tiny Tank, but with a bunch of exciting new characteristics!

1. Heavy Tank, as the name suggests, has a larger body.
2. It moves more slowly and fires less frequently. (The fire interval is 2 seconds)
3. It has more hitpoints (maximum health = 150), which means it's stronger.
4. The cannonball it fires deals more damage (1.5x).
5. Last but not least, it can "SuperFire"! (which means "放大招" in Chinese) If the user right clicks the mouse, the Heavy Tank can fire a Big Cannonball (a new bullet), which is not only bigger but also deals 4x damage. The SuperFire interval is 10 seconds.
   
   Close Issue 每次发射时有概率释放大招 #226