设置navigation完全透明效果（How to make navigation bar transparent in iOS 7）

原文地址：http://stackoverflow.com/questions/20319439/how-to-make-navigation-bar-transparent-in-ios-7

[self.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
self.navigationBar.shadowImage = [UIImage new];
self.navigationBar.translucent = YES;