PSR-0 Autoloader

Provides a PSR-0 compatible autoloader when composer is not desired or is not available on a project.

```php
<?php
require dirname(__FILE__) . '/path/to/Purl/Autoloader.php';
\Purl\Autoloader::register();
```

Also adds tests to accompany the Autoloader class.

I normally use Composer whenever possible, but there was a specific situation where I wanted to use the Purl library, but did not have the time to add composer to the deploy and build process. It also helps when packaging with a plugin that is just simpler to remain self-contained.

The Purl library is really handy by the way, thanks! Cheers!