JSON implementation for rest handler

See https://github.com/5HT/n2o/pull/38

https://github.com/b0oh/n2o_rest – example application
https://github.com/b0oh/n2o_rest/blob/master/apps/web/src/users.erl – rest module

soon https://github.com/b0oh/n2o_rest will be moved to n2o_sample