Edit> Transform> Skew, Distort, Perspective
(drag the corner squares)
you will need to turn the background into a layer (Option+doubleclick on layer in pallet)
Command+0 (fit screen)
Command+- (zoom out)
F (key to move corners outside of image)
Edit> Transform> Skew
My guess is this stirs up the pixels pretty bad so you will need a bigger file and try to do all the corrections in one move...setting guides or grid may also be good.