add maintenance mode

Maintenance mode will display a "System down for maintenance" message to all who visit. It can be set via

```php
set_config('system', 'maintenance', 1);
```

and turned off with

```php
set_config('system', 'maintenance', 0);
```

Or it can be done from the command line, using

`php util/maintenance.php`

to turn on maintenance mode, and

`php util/maintenance.php 0`

to turn it off