- (void)setTitle:(NSString *)title forState:(UIControlState)state;
设置按钮的文字
- (void)setTitleColor:(UIColor *)color forState:(UIControlState)state;
设置按钮的文字颜色
- (void)setImage:(UIImage *)image forState:(UIControlState)state;
设置按钮内部的小图片
- (void)setBackgroundImage:(UIImage *)image forState:(UIControlState)state;
设置按钮的背景图片
设置按钮的文字字体（需要拿到按钮内部的label来设置）
btn.titleLabel.font = [UIFont systemFontOfSize:13];
- (NSString *)titleForState:(UIControlState)state;
获得按钮的文字
- (UIColor *)titleColorForState:(UIControlState)state;
获得按钮的文字颜色
- (UIImage *)imageForState:(UIControlState)state;
获得按钮内部的小图片
- (UIImage *)backgroundImageForState:(UIControlState)state;
获得按钮的背景图片