code: function create(req, res, next) {
    var body = req.body;
  
    db.users.create(body, function(err, user) {
      if(err) { return next(err); }
      res.json(201, user);
    });
  }
docs: URL: /users/
  
  Method: POST
  
  Status codes:
  
  * `201` if user was created successfully
  * `409` if a user with the same `username` already exists