self.navBar = [[UINavigationBar alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, NAV_BAR_HEIGHT)];
[self.navBar setBackgroundImage:[UIImage imageWithColor:[UIColor clearColor]] forBarMetrics:UIBarMetricsDefault];
[self.navBar setBackgroundColor:UIColorFromRGBA(0xFF1E1E1E)];
self.navBar.shadowImage = [[UIImage alloc] init];
使用UIImage+Color2Image.h将透明色值转换为图片，
设置shadowImage去掉底部lineUINavigationController设置透明背景
原文：http://my.oschina.net/redhouse/blog/468573
