title: Does LoopbackJS automatically create createdAt and modified fields on a new model creation? How to enable?
body: <p>I'm curious, does LB <em>automatically</em> create <code>createdAt</code> and <code>modified</code> fields on new model creation, like RoR does? LB docs says that it should, but when I try to create a new model, LB doesn't create those fields. Here is my script to create a new model and a new document in a MongoDB collection:</p>
  
  <pre><code>'use strict';
  
  var server = require('./server');
  var ds = server.dataSources.mongodb;
  var collections = ['user'];
  
  
  ds.automigrate(collections, function(er) {
      if (er) throw er;
      console.log('== Looback tables [' + collections + '] created in ', ds.adapter
          .name);
  
      server.models.user.create([{
          username: 'admin',
          password: "admin",
          email: 'admin@gmail.com',
      }], function(err, newUser) {
          if(err) {
              console.error("== Create ERROR ==:\n\r", err);
              ds.disconnect();
              process.exit(1);
          }
          console.log('== newUser ==:\n\r', newUser);
          ds.disconnect();
          process.exit();
      });
  });
  </code></pre>
  
  <p>And this is the document created. No <code>createdAt</code> and <code>modified</code> fields:</p>
  
  <pre><code> [ { username: 'admin',
      password: '$2a$10$.IqOjq1EEU1s1zEeb7IO7.tLAa0HljgS2frAR77AsoB8jgfZjWfi6',
      email: 'admin@gmail.com',
      id: 5642e5ecbf5d88742390971f } ]
  </code></pre>
  
  <p>How to enable that feature? Or should I explicitly pass those values every time I create a new model or modify an existing model? Like this:</p>
  
  <pre><code>server.models.user.create([{
      username: 'admin',
      password: "admin",
      email: 'admin@gmail.com',
      createdAt: new Date(), &lt;--
      modified: new Date() &lt;--
  }]
  </code></pre>