Use the composer autoloader and clean up white space

By adding the class to the autoload class map, when people install via composer and require the vendor/autoload file - they don't need to also require the robots file. 

eg;

```
<?php
require 'vendor/autoload.php';
$parser = new RobotsTxtParser(file_get_contents('http://example.com/robots.txt'));
```

Which is handy if they have several other packages installed via composer, means we can just leverage that.

Also cleaned up some white space, had a mix of 3 and 4 space indentation.