Allow selecting the TranslatorAwareTreeRouteStack via configuration

Per the mailing list, there's currently no way to opt-in to using the TranslatorAwareTreeRouteStack without overwriting the "Router" service. This PR fixes that situation, by allowing a new configuration item inside the router configuration, "router_class", which allows you to specify the router to use. The factory will then invoke that class' `factory()` method to retrieve an instance. This can be done for either console or HTTP router types.

```php
return array(
    'console' => array(
        'router' => array(
            'router_class' => 'My\Custom\Console\Router',
            'routes' => array( /* ... */ ),
        ),
    ),
    'router' => array(
        'router_class' => 'My\Custom\Http\Router',
        'routes' => array( /* ... */ ),
    ),
);
```