Delete obsolete game.html
Add MissileTower, a tower conterpart of MissileEnemy
Add BattleShipEnemy and BattleShipTurret

Also expanded range of bullets for making the battle ship turrets
effective from far range.
Add endurance mode, which never ends until all towers are destroyed
Prevent missile trails from remaining if the missile lives long
Add life parameter to Bullet to prevent missiles from living forever