backButton更换图片  backbutton图片尺寸的话 42*42
写法一：
[self.navigationBar setBackIndicatorImage:[UIImage imageNamed:@"Nav_Back"]];
[self.navigationBar setBackIndicatorTransitionMaskImage:[UIImage imageNamed:@"Nav_Back"]];
写法二：
 UIImage *backButtonImage = [[UIImage imageNamed:@"Nav_Back"] resizableImageWithCapInsets:UIEdgeInsetsMake(0, 0, 0, 0)];
[[UIBarButtonItem appearance] setBackButtonBackgroundImage:backButtonImage forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];


去除backButton文字
[[UIBarButtonItem appearance] setBackButtonTitlePositionAdjustment:UIOffsetMake(NSIntegerMin, NSIntegerMin)
                                                     forBarMetrics:UIBarMetricsDefault];