Class Site, getMenu and unit test refactoring

Introduced class Site, actually only the Stub version, but anyway :)
Implemented is PageRenderer->getMenu, which uses the method from
Site class. getMenu now loads strings from all pages (one by one),
probably this place would need some sort of optimization. Also the
menu tag is not finished, it does not contain any lactually only the <nav>
element.
Merge branch 'feature-get-menu'