Change hard coded "app/" to app_path()

When someone changes the name of the "app" directory, the generator gives an error because the "app" directory was hard coded in the files. I've changed this to app_path() so that it will always find the right directory name. For example:

```php
array('path', null, InputOption::VALUE_OPTIONAL, 'Path to views directory.', 'app/views')
```

changed to:

```php
array('path', null, InputOption::VALUE_OPTIONAL, 'Path to views directory.', app_path() . '/views')
```