working on animation
Changed several major aspects of Entity class:
	Uses functors instead of inheritance.
	Has all necessary attributes for rendering and animation.
	Now inherits directly from sprite instead of having sprite member.

Changed EntityManager to use vector<unique_ptr<Entity>> instead of map.

Made all other systems compatible with new Entity class.

Note:
	Rendering and animation is to be moved from Entity to a seperate
	system that iterates renderables and updates their frame if they
	are animated.

As of this commit, the code compiles without error or warnings. The
next step is reimplementing all tests.
Simplified Entity System.

Removed entity inheritance system.

Moved animation methods from Entity to SpriteManager.

Changed Entity to have all necessary attributes for animation and
rendering.

Made Entity subclass of sf::Sprite.
updated CMakeLists