Documenting the optional default value parameter

Each of these:

```php
$app->request->get()
$app->request->post()
$app->request->put()
```

Accepts an optional second parameter to specific a default value.  This is currently undocumented, so I documented it.  Evidence:

https://github.com/codeguy/Slim/blob/master/Slim/Http/Request.php#L215