added changed information to text element so we can mark it as inserted or deleted #350 



Example usage:
$phpWord = new \PhpOffice\PhpWord\PhpWord();
$section = $phpWord->addSection();
$text = $section->addText('Hello World!');

$text = $section->addText('Hello World!');
$text->setChanged(\PhpOffice\PhpWord\Element\ChangedElement::TYPE_INSERTED, 'Fred', time()-1800);

$text = $section->addText('Hello World!');
$text->setChanged(\PhpOffice\PhpWord\Element\ChangedElement::TYPE_DELETED, 'Barney', time()-3600);