Created SceneManager class.

This class will handle the different Scenes in the game.
Expaneded functionality for SceneManager.

Also it has been integrated with Game class and created TestScene in the test game.
Merge branch 'clean-up' of https://github.com/Poles/Poles into clean-up

Conflicts:
	include/engine/Game.h
	include/engine/Scene.h
	src/engine/Game.cpp