Added ParallaxingBackground.cs

Added the .cs file and added a skeleton for the class.
Updated ParallaxingBackground.cs Initialize()

Updated the Initialize() function to set up an background instance
Updated Update() in ParallaxingBackground.cs

I added the code in Update() so that the background will scroll
Updated ParallaxingBackground.cs and Game1.cs

Finished the Update() method for the moving background, and updated Game1.cs to use the moving background