Add a function for retrieval of charset

[cwdgservices]

I use to use an old version of w3c validation system and I record the Charset. I thought It would be beneficial to your code to make it available.

/**
* Get Charset
*
* Returns the charset found in the document.
*
* @return str
*/
public function getCharset()
{
return (int)$this->getPath('//m:charset');
}