Refined Collision Detection

-Added collisionCenter to offset the collision box.
-Rewrote collision rectangle to account for collisionCenter.
-Tweaked collision detection.
-Adjusted constructors for sprites.
-Changed Player from AnimatedSprite to Creature.
-Added center method for sprites.  This returns a Vector2 that is the
drawing center based off the resolution and size of the sprite
texture/frame size.  Using this as the drawing position should draw the
sprite in the center of the screen.