Make GitHubContributor extend GitHubUser

`GitHubContributor` should extend `GitHubUser` instead of just `GitHubObject`.

This means that you can access details about the contributor as a user, such as their username and avatar.

Currently all you can do is list the number of contributions:

```php
$contributors = $client->repos->listContributors($user, $repo);

foreach ($contributors as $contributor) {
    echo $contributor->getContributions() . "<br>";
}
```

Whereas with the change you can access their user information too:

```php
$contributors = $client->repos->listContributors($user, $repo);

foreach ($contributors as $contributor) {
    echo $contributor->getLogin() . ': ' . $contributor->getContributions() . "<br>";
}
```