Added support for scaling images inside SDWebImage. In case scaling is desired for the image, we will query the disk/memory for the scaled image. If no scaled image is found, we will query the original image on disk/memory, scale it, save the scaled image to disk and memory and return it. If there is no image in the cache, we will download the original image, scale it, save the original image to disk, save the scaled image to memory and disk and return the scaled image.