调整leftBarButtonItem的位置

参考：http://bbs.csdn.net/topics/390609070
UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 43, 32)];
[button setImage:[UIImage imageNamed:@"common_back_btn_n.png"] forState:UIControlStateNormal];
[button setImage: [UIImage imageNamed:@"common_back_btn_h.png"] forState:UIControlStateHighlighted];
//使图片变形（压缩或延伸）
[button setImageEdgeInsets:UIEdgeInsetsMake(0, -15, 0, 15)];
//使button上的文字的位置改变

// [button setTitleEdgeInsets:UIEdgeInsetsMake(400, 200, 0, 0)];
//使button上的内容发生变化（图片的形变+文字的位置的变化）

// [button setContentEdgeInsets:UIEdgeInsetsMake(12, 0, 18, 0)];

[button addTarget:self action:action forControlEvents:UIControlEventTouchUpInside];

UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithCustomView:button];
self.navigationItem.leftBarButtonItem = backButton;

注意
：在给button设置图片时，必须是setImage,否则的话
setImageEdgeInsets：方法没有效果。