Implement support for JWT

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=7671

Reported by Martin Karlgren marty@roxen.com

Implement Pike support for JSON Web Tokens (encoding and decoding), see http://jwt.io/ and RFC 7519.

The decode function should have support for specifying allowed algorithms, defaulting to some reasonable value (see discussion on possible vulnerabilities: https://auth0.com/blog/2015/03/31/critical-vulnerabilities-in-json-web-token-libraries/)