Updated to avoid full tree traversal.
Also noticed ```activateItems``` didn't do anything so that's been fixed.

Based off the method in ```Menu.php``` I've updated ```Nav``` so it will check if sub menus are active or not.

fixes #2466

Fixes #1881: Improved `yii\bootstrap\NavBar` with `containerOptions`, `innerContainerOptions` and `renderInnerContainer`

Removed edited author ref.

Updated tabs

Corrected the example and doc to highlight allowed dropdown support in Bootstrap 3.