Implement tables and expose styles in addHtml

This pull request ehances the ```\PhpOffice\PhpWord\Shared\Html::addHtml``` function to :

- Be able to insert tables
- Set the default styles
- Use colspan for table cells (sorry, I didn't see any reliable way to implement rowspan fonctionnality at the moment, mainly due to the lack of end of node event in the implementation)

It should also help with issue #417