feat: newArticleGroup API call

[roydejong]

This adds support for the newArticleGroup API call.

Example:

<?php

/**
 * @var $client MplusQAPIclient
 */
$response = $client->newArticleGroup([
  'name' => 'My group',
  'text' => 'Extra text'
]);

var_dump($response);
// array (size=1)
//  'groupNumber' => int 123