'.carousel ing' was replaced with '.carousel-inner > .item > img' in
3.0 and now loses the specificity war. To make carousel images 500px
tall, match specificity to make carousel.css win.
A .navbar-wrapper followed by a .carousel works well in this example,
but when the navbar is not followed by a carousel, content flows
underneath the navbar because it is absolutely positioned.
Letting .navbar-wrapper be part of the flow and moving the negative
margin to the .carousel to bring it underneath the navbar makes these
example styles suitable for pages without carousels.
