uinavigator item 位置调整

转自：http://stackoverflow.com/questions/18914812/how-to-edit-empty-spaces-of-left-right-uibarbuttonitem-in-uinavigationbar-ios

UIBarButtonItem *negativeSpacer = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];

negativeSpacer.width = -16; // it was -6 in iOS 6

[self.navigationItem setLeftBarButtonItems:@[negativeSpacer, requiredButton /* this will be the button which you actually need */] animated:NO];