Update navigation elements to correctly display

It looks like between Bootstrap 3.0.0-rc1 and final (see 71483417194a873a540be60529ae49c7ad2b36f8), there was a class name change from `nav-collapse` to `navbar-collapse`.  This change addresses that (making navigation visible again) and changes the style of the collapsed navigation button.
Merge pull request #13 from openlayers/nav

Make the navbar elements visible.