Some fixes and information about the default configs and views.

* Documentation of the `view` config key, and include it in `config/menu/navbar.php`;
* Change the default config file to `simple` (which exists) instead of `bootstrap` (which doesn't); and
* Switch to full PHP opening tags instead of short tags (as per [PSR-1](http://www.php-fig.org/psr/psr-1/)).