Updates allowing parser to handle single label hosts.

Fixes #22.

Verified fix with updated tests, and additionally with the following script:
```php
<?php

require_once __DIR__ . "/vendor/autoload.php";

use Aura\Uri\Url\Factory as UrlFactory;
use Aura\Uri\PublicSuffixList;

$psl = new PublicSuffixList(require_once __DIR__ . '/data/public-suffix-list.php');
$url_factory = new UrlFactory($_SERVER, $psl);
$url = $url_factory->newInstance('http://localhost/');
echo $url->__toString() . "\n";
```

Output:

``` bash
$ php -f issue-22.php
http://localhost
```