判断设备和retina

#define isPad (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)#define isRetina ([UIScreen instancesRespondToSelector:@selector(currentMode)] ? CGSizeEqualToSize(CGSizeMake(640, 960), [[UIScreen mainScreen] currentMode].size) || CGSizeEqualToSize(CGSizeMake(1536,2048), [[UIScreen mainScreen] currentMode].size) : NO)