refer to $app as $this inside a router

## as a sequence to this [pull request](https://github.com/codeguy/Slim/pull/660)

### Instead of repeating the keyword use everywhere like that....

```php
$app = new \Slim\Slim();

$app->get('/',function ()  use ($app) {

});

$app->group('/api',function ()  use ($app) {

    $app->get('/',function ()  use ($app) {

    });

});
```

### We can just use $this:

```php
$app = new \Slim\Slim();

$app->get('/',function () {

});

$app->group('/api',function ()  {

    $this->get('/',function ()  {
          $this->halt(404);
    });

});
```