The AbstractPowerUp now keeps track of effectDuration and when the effect is active.

The class comes with two abstract methods that needs to be added into
the specific powerup class, what happens when it's applied and what
happens when it runs out.
Added two (2) powerups: FireRate and Speed.
The tank now keeps more accurate track of when a powerUp is applied, and takes care of it.
Changed the fireRate in flamethrowerTurret to 32 (from 1) so that the fireRatePowerUp will have an effect.