Support background image cropping in AFImageResponseSerializer

I added support for background image cropping to AFImageResponseSerializer in my local copy of the AFNetworking project
i.e.
Developers can specify a target CGSize when they download images via this new UIImageView::setImage method:

@interface UIImageView (AFNetworking)
- (void)setImageWithURLRequest:(NSURLRequest*)urlRequest
                    targetSize:(CGSize)targetSize
              placeholderImage:(UIImage*)placeholderImage
                       success:(void (^)(...))success
                       failure:(void (^)(...))failure;
@end

If a developer uses this method, then
  The AFImageResponseSerializer crops downloaded images to the targetSize before returning it
Else
  The AFImageResponseSerializer just returns the full image like it's been doing