Use correct dropdown header class in sticky-footer

"nav-header" does not exist in Bootstrap. The class used for headers
in dropdowns is "dropdown-header".
Enable dropdown in sticky-footer with Bootstrap JS

The sticky-footer example uses a drop down in its navbar, and dropdowns
require Bootstrap's JS to work.

Use the minified version of Bootstrap since this example is about the
CSS and not about the JS.
Merge pull request #24 from ssorallen/sticky-footer-js

Sticky footer example JS and navbar dropdown style