随机色产生的宏

平时开发容易忘记小知识点，防止以后忘记：

#define random(r, g, b, a) [UIColor colorWithRed:(r)/255.0 green:(g)/255.0 blue:(b)/255.0 alpha:(a)/255.0]#define randomColor random(arc4random_uniform(256), arc4random_uniform(256), arc4random_uniform(256), arc4random_uniform(256))