let's follow laravel naming conventions so PayumFacade -> Payum

With this change, all we need in the `app.php` is:

````
'aliases' => array(
    // ....
    'Payum'           => 'Payum\LaravelPackage\Facades\Payum'
);
````

Then we can use it in our code as `Payum::method()` instead of `PayumFacade::method()`