Reusable configuration keys

[cmygeHm]

1. It's not so useful to initialize client with string configuration keys, as your examples:

    $configuration = new \ElasticEmailClient\ApiConfiguration([
        'apiUrl' => 'https://api.elasticemail.com/v2/',
        'apiKey' => 'yourApiKey'
    ]);

More useful:

    $configuration = new \ElasticEmailClient\ApiConfiguration([
        \ElasticEmailClient\ApiConfiguration::API_URL => 'https://api.elasticemail.com/v2/',
        \ElasticEmailClient\ApiConfiguration::API_KEY => 'yourApiKey'
    ]);

2. I think it is better to pass this options via setters and not to do them as required arguments of constructor:

$config = new ApiConfiguration();
$config
            ->setApiUrl($apiUrl)
            ->setApiKey($api);

If you do not agree with me I can remove this commit 478eb73