Added support for QR codes.
[GH-44] The X coordinate returned by $text->position() or (deprecated)
$text->textpos() was incorrect when read after being set twice (report by
[GH-45] Renamed transform's "relative" option to "repeat" and fixed the
documentation describing what it does (report by Johan Vromans).
[GH-49] Fix page_layout storing its value as page_mode (report by Johan
[GH-50] Fix $pdf->viewer_preferences to return the same values it accepts
for non_full_screen_page_mode (report by Johan Vromans).
Minor doc clarifications and improved error messages.