Use reviver and replacer in json type

[sitegui]

Somehow tell the lib a replacer function to JSON.stringify and a reviver for JSON.parse

This would help with https://github.com/clubedaentrega/log-sink/issues/3

Note that message property is non-enumerable. Related discussion at http://stackoverflow.com/questions/18391212/is-it-not-possible-to-stringify-an-error-using-json-stringify