Fix incorrect image size between windows and mac.

[hskrtich]

See Issue #224

[jorisvm]

I had a simular problem in a project today and ended up converting the pixels to points. What I did was add the folowing lines to: Writer/Word2007/Element/Image.php in the writeImage method:

<?php

// Force styles in points (Google drive and word for MAC don't like pixels for images)
        $style->setUnit('pt');
        $style->setWidth(number_format(\PhpOffice\PhpWord\Shared\Converter::pixelToPoint($style->getWidth()), 2, '.', ''));
        $style->setHeight(number_format(\PhpOffice\PhpWord\Shared\Converter::pixelToPoint($style->getHeight()), 2, '.', ''));
?>