重載UIImage的方法以方便使用@2x圖片

參考：Custom UIImage Subclass To Support @2x Retina Devices

-(id)initWithContentsOfFile:(NSString *)path  
{  
    if([UIScreen instancesRespondToSelector:@selector(scale)] && [[UIScreen mainScreen] scale] == 2.0 )  
    {  
        NSString *path2x = [[path stringByDeletingLastPathComponent]  
                stringByAppendingPathComponent:
                [NSString stringWithFormat:@"%@@2x.%@", 
                [[path lastPathComponent] stringByDeletingPathExtension],
                [path pathExtension]]];  
   
        if ([[NSFileManager defaultManager] fileExistsAtPath:path2x])
             return [super initWithContentsOfFile:path2x];  
    }  
   
    return [super initWithContentsOfFile:path];  
}