* fixed rotation of sprites.
* scaled everything up, we are using 700x600 @ 1x scaling now.
* added a shaking effect when the player is hit.
* added a white-overlay sprite when an enemy unit is hit
* added a particle emitter class that spawns particles when enemy dies.
* implemented a pause state.
* added TweenText to wave creation.
* added TweenBox to wave creation.
* Tween now finishes before wave is created.
* added Credits, Credits animation and Credits pickup sound.