code: function(username, password, attrs, options) {
          attrs = attrs || {};
          attrs.username = username;
          attrs.password = password;
          var user = AV.Object._create('_User');
          return user.signUp(attrs, options);
        }
docs: Class Methods 
  Signs up a new user with a username (or email) and password.
  This will create a new AV.User on the server, and also persist the
  session in localStorage so that you can access the user using
  {@link #current}.
  
  @param {String} username The username (or email) to sign up with.
  @param {String} password The password to sign up with.
  @param {Object} [attrs] Extra fields to set on the new user.
  @param {AuthOptions} [options]
  @return {Promise} A promise that is fulfilled with the user when
  the signup completes.
  @see AV.User#signUp