RKCropImageController 介绍
RKCropImageController 是一个实现了图片剪切的 iOS 控件，支持缩放。
使用方法：
// in header
#import "RKCropImageController.h"
// in code
RKCropImageController *cropController = [[[RKCropImageController alloc] initWithImage:imageView.image] autorelease];
cropController.delegate = self;
[self presentModalViewController:cropController animated:YES];
// delegate callback
-(void)cropImageViewControllerDidFinished:(UIImage *)image{
imageView.image = image;
}