Added a new weapon - Shockwave

At first the weapon launches a single projectile that after a short
peroid of time sends out a shockwave.
Decreased the spread of SlowspeedyTurret
Implemented the DamagePowerUp

 it doubles the damage of the projectiles
Added a updateEffect method to AbstractPowerUp

The method runs everytime the object (entity) is updated.
Removed a comment
Removed unnecessary code
Made it easier to change turret, also removed a comment
Added some simple images for the powerups
Play spawns three powerups at startup, and also checks if entities are active before drawing
PowerUp need a position when created and now register collision (pickup)
Removed unnecessary code
Removed some code in setCurrentPowerUp (AbsTank) that shouldn't be there
Fixed a bug where you could pick up the powerup twice
Updated the powerup images
Edited the size of the powerups
Changed the name of the package "powerup" to "powerups"
The firerate powerup now also works if the turret is changed
EnterState changed to PLAY
Changed the positions that PowerUps spawn at
Moved a comment