Create Statocles::Navigation object

[preaction]

A Navigation object is like a link, but it also allows for child links to allow for multi-level navigations (dropdown menus, tabbed navigation, or just categorized lists of links).

• [ ] Create the Statocles::Navigation object
• [ ] The site's nav attribute should be this, not plain Link objects.
• [ ] The Bootstrap theme should allow for a single level of dropdown (as Bootstrap 3 only supports that) in the main nav. The default theme should allow for no dropdown at all.