tableview偏移
方法一：改变edgesForExtendedLayout
self.edgesForExtendedLayout = UIRectEdgeNone;
将edgesForExtendedLayout属性设置为UIRectEdgeNone，这样布局就是从导航栏下面开始了
方法二：导航栏半透明属性设置为NO
@property(nonatomic,assign,getter=isTranslucent) BOOL translucent NS_AVAILABLE_IOS(3_0) UI_APPEARANCE_SELECTOR; // Default is NO on iOS 6 and earlier. Always YES if barStyle is set to UIBarStyleBlackTranslucent
在iOS 6之前（包括iOS 6）translucent默认就是NO，在iOS 7就默认是YES了。
self.navigationController.navigationBar.translucent = NO;
将导航栏的半透明属性关闭掉，布局也是从导航栏下面开始了。
判断ios系统是不是7.0
if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7.0) {
self.edgesForExtendedLayout = UIRectEdgeNone;
}