Added a simple validation for new app name

Currently there are no validation for app name. For example when user entered like ```Vendor;Product``` or like this ```Vendor§§Produc``` name, Laravel gives syntax error.

I added a simple validaton for this issue. And PR contains a little improvement for Composer's namespace about this issue.
The should be a new line before the return. Like this:

```php
    /**
     * Check the app name contains only valid characters.
     *
     * @return bool 
     */
```