Added Function: Update User

Function to update user information.

**Example**
```php
use GorkaLaucirica\HipchatAPIv2Client\Auth\OAuth2;
use GorkaLaucirica\HipchatAPIv2Client\Client;
use GorkaLaucirica\HipchatAPIv2Client\API\UserAPI;

$auth = new OAuth2('APITOKENHERE');
$client = new Client($auth);
$userAPI = new UserAPI($client);

$user = $userAPI($userId);
$user -> setTitle("newTitle");
$userAPI -> updateUser($user);
```