Better Retina Support

Hi,

I made a few changes:

* Fixed project build paths so it builds.
* Changed the extension spritesWithContentsOfFile: to be a class method so you don't need to alloc a UIImage just to call it.
* Fixed so Retina (@2x) images are properly scaled back down. Otherwise they appear twice the size you wanted them to be. This is easily seen in the SampleProject which showed the hill,tree,cloud images twice as big on the Retina screens.