Implement listing all labels, and creating labels

I'm not sure if I've missed a different interface - but this allows the following syntax to work, [from the docs](https://github.com/KnpLabs/php-github-api/blob/master/doc/issue/labels.md#list-project-labels):
```php
$labels = $client->api('issue')->labels()->all('KnpLabs', 'php-github-api');
```

Also add an interface for creating labels.