adding naive/trivial support for cdata

This makes it simple to invoke XML\Writer::write() on an array with cdata
things like so:

```php
$writer = new Sabre\XML\Writer();
$writer->openMemory();
$writer->setIndent(true);
$writer->startDocument('1.0', 'UTF-8');
$writer->write(
  array(
    'my-sweet-node' => array(
      'a-cdata-node' => new Cdata('some cool text in a cdata')
    )
  )
);
```

```XML
<?xml version="1.0" encoding="UTF-8"?>
<my-sweet-node>
 <a-cdata-node><![CDATA[some cool text in a cdata]]></a-cdata-node>
</my-sweet-node>
```