added new file "pickup.php" to the plugin

the reason I put this addition in is that I want to be able to get the certificate from an external source (either a curl process on another server/portal, or a hyperlink from an e-portfolio page, etc).

The system is picking up via the certificate's issue id, so it's a reasonably random string that's hard to guess. But there might need to be better thought put into how to secure the process more. The page isn't actually put into a session-authenticated state, so you can't spoof moodle in to thinking you're logged on as the user who you are picking up, if that's something.