Add difference percentage

[viethg1995]

Current result:
{
width: number;
height: number;
imagesAreSame: boolean;
diffCount: number;
}

New result:
{
width: number;
height: number;
imagesAreSame: boolean;
diffCount: number;
percentage: number Float - Normalized total difference
Value can range from 0.00 (no difference) to 1.00 (every pixel is different)
}