Dev rework answers PDFs and add header

New PDF format for single answer export and new header for PDF admin answers export. New config params are added to "Global Configuration -> Presentation" tab including an option to enable/disable headers in PDFs (Version 2.06).

PDF generation is broken after this patch but it was already broken in 2.06.
The error is the same: pdfHelper.php:64: $pdflang = new Limesurvey_lang($language);