Method chaining in Parser

I've simply added the line ```return $this;``` in some ```Parser``` methods for method chaining like this:

```php
$parser = new JBBCode\Parser();
    ->addCodeDefinitionSet(new JBBCode\DefaultCodeDefinitionSet())
    ->addCodeDefinitionSet(new SpecialCodeDefinitionSet());

$text = "The bbcode in here [b]is never closed!";
print $parser->parse($text)->getAsBBCode();

$text = "[u]Another bbcode example![/u]";
print $parser->parse($text)->getAsHtml();
```

I also fixed the documentation in ```Parser``` for some methods.