Fix attaching routers

I didn't find appropriate way to provide service definitions in
previous implementation. This uses only service name.
Example implementation of IRouterProvider:
```php
class AppExtension extends Nette\DI\CompilerExtension implements \Rixxi\Modular\DI\IRouterProvider
{
    public function loadConfiguration()
    {
        $this->getContainerBuilder()->addDefinition($this->prefix('myRouter'))
            ->setClass('App\MyRouter');
    }

	public function getRouters()
	{	
		return array(
			$this->prefix('@myRouter'),
		);
	}
}
```