Option added for alwaysDropdown to enable overriding of MQ breakpoint.

Many website navigation experiences employ mobile-style nav for all device experiences (e.g. http://dockyard.com/). This PR adds support for such an experience by exposing an `alwaysDropdown` option.

By default, `alwaysDropdown` is set to `false`, which brings about no change to responsive-nav.js. However, if the developer sets `alwaysDropdown` to `true` then the rules that define the navigation layout in the MQ in `responsive-nav.css` will not take effect. The result is mobile-style navigation no matter how wide the viewport is.

Backwards-compatible. Tests, demos, and version changed accordingly.