New function pdf_underlay_stamp()

[trevorld]

• New function pdf_underlay_stamp() has same structure as pdf_overlay_stamp().
• I renamed cpp_pdf_overlay() to cpp_pdf_stamp() and gave it a underlay boolean flag. If true we underlay the stamp, if false we overlay the stamp. The key difference in logic is as follows:

        if (underlay) {
          ph.addPageContents(
            QPDFObjectHandle::newStream(&inpdf, content + "\nq\n"), true);
          ph.addPageContents(
            QPDFObjectHandle::newStream(&inpdf, "\nQ\n"), false);
        } else { // overlay
          ph.addPageContents(
            QPDFObjectHandle::newStream(&inpdf, "q\n"), true);
          ph.addPageContents(
            QPDFObjectHandle::newStream(&inpdf, "\nQ\n" + content), false);
        }

closes #31