use JSON instead of xml when communicating with URKUND

[danmarsden]

When the Urkund api was first created xml was the only option, but now it supports JSON we'd like to convert to using json for validating the response in urkund_get_score and plagiarism_urkund_update_allowed_filetypes

To do this we need to add a new accept header to the curl calls eg here: https://github.com/danmarsden/moodle-plagiarism_urkund/blob/master/lib.php#L1420

should now be this: $headers = array('Accept: application/json', 'Accept-Language: '.$plagiarismsettings['urkund_lang']);

and then instead of using simplexml we can use json functions.

Also - on an error state, we store the response in the errorresponse field, here: https://github.com/danmarsden/moodle-plagiarism_urkund/blob/master/lib.php#L1456

and then parse that when showing error data in the get_links() table https://github.com/danmarsden/moodle-plagiarism_urkund/blob/master/lib.php#L251