Extend HTML output for pagebreaks for PDF generation

Make HTML output of pagebreak compatible for the different PDF writers by combining:
http://mpdf1.com/manual/index.php?tid=110 (tested and working)
and
https://code.google.com/p/dompdf/wiki/FAQ (tested and working)
and
http://chandeshparekh.com/css-to-create-new-page-in-php-tcpdf-pdf/

src/PhpWord/Writer/HTML/Element/PageBreak.php

@@ -24,4 +24,13 @@ namespace PhpOffice\PhpWord\Writer\HTML\Element;
  */
 class PageBreak extends TextBreak
 {
+    /**
+     * Write page break
+     *
+     * @return string
+     */
+    public function write()
+    {
+        return '<pagebreak style="page-break-before: always;" pagebreak="true"></pagebreak>';
+    }
 }