Changed the powerups to be spawned, for testing
Moved the effectActive so that powerups deactivates correctly
Simple null check to avoid exceptions when a tank dies
Removed an unnecessary sysOut
The shield powerup is now correctly deactivated

The updateEffect now checks if the shield is active. If it isn't
active, the powerup itself deactivates, otherwise it moves the shield.
The shield now keeps track of the tank it is shielding and has health

The purpose of this is that the projectiles fired from the tank, that
has the shield activated, shouldn't damage the shield.