(see Desc) Added World.h/cpp

Alright, almost there; all that's left is to integrate this code into
our game so it's actually running when we call Galaga::run().
Differences between my code and the textbook are the following:
* Entity now inherits from SceneNode
* mScrollSpeed is irrelevant, as the screen will not be moving.
* mTexture of type sf::Texture is used over a TextureHolder object.
* A shared pointer mTexturePtr is passed to new Aircraft objects.
* A shared pointer spriteMapPtr is passed to new Aircraft objects.
* New function prepareSpriteMap() to instantiate the spriteMap pointer.
* mWorldBounds remain within our playing field.
* mWorldView's center is concretely set as the center of our playing
field.
* mWorldView does not ever move.
* Our loadTextures() method directly loads the texture (no
TextureHolder).
* No background sprites are loaded (yet).
(see Desc) Minor Tweaks

* Cleaned up #include block for Aircraft.h, Galaga.h and SceneNode.h/cpp
* Defined default SceneNode constructor (intended for previous commit)
* Added a try/catch in Galaga::main()
* Nudged over the SFXText
* Corrected the scoreDisplay