fix 404 page if 'admin' or 'admin/' was requested

I've just downloaded your plugin and hit 404 when went to `admin/` (even worse, I encountered exception when typed `admin` without slash). This fix fetches first page URL from `settings['nav']` list, so that it's opened by default. There was a comment _redirect missing pages to the home page_, but I don't think it works (it doesn't work for me).

Alternatively, there could be 'index' method in Pages (so here you'd have `if (!isset($uri[1]) || $uri[1] === '') $url[1] = 'index';`), which could default to some arbitrary page or first from nav, like here.