Removed unnecessary code in the delete and add instance functions
CONTRIB-4111 - Ensure certificate dates generated use Moodle's userdate function for consistency
CONTRIB-4089 - Create a directory that is writeable so that TCPDF can create temp images
CONTRIB-4089 - Changed usage of TCPDF to PDF

The PDF class extends the TCPDF class, so the same functionality is accessible.
Fixed issue with defaults not being set when adding a new certificate
Increased version number
Merge changes from upstream master (conflicts resolved)