Fix bug with "isprint" being different on Windows.

http://www.cpantesters.org/cpan/report/d6438b68-6bf4-1014-8647-737bdb05e747
Add json_diagnostics variable which switches on error messages in JSON.

Version bump & changes update for new version.

Now second half of surrogate pair error dealt with by
"in_surrogate_pair" expected value, also inserting into literals.