title: adding a createdBy field to the User model in keystone.js
body: <p>I got the 'createdBy' field added to the model but it allows the admin to select all users as the 'createdBy' user.  I want this field to be auto populated with the admin that is currently logged in and can't seem to get it to work.  </p>
  
  <p>Ideally this wouldn't appear in the UI at all but just be stored when the user is saved.</p>
  
  <pre><code>User.add({
      name: { type: Types.Name, required: true, index: true },
      email: { type: Types.Email, initial: true, required: true, index: true },
      company: { type: String, required: true, index: true, initial: true },
      phone: { type: String, required: true, index: true, initial: true },
      password: { type: Types.Password, initial: true, required: true },
      createdBy: { type: Types.Relationship, initial:true, required:true, ref: 'User' },
      createdAt: { type: Date, default: Date.now }
      }, 'Permissions', {
        level : { type: Types.Select, numeric: true, options: [{ value: 1, label: 'User'   }, {     value: 2, label: 'Group Administrator' }, { value: 3, label: 'System Administrator' }] }
      },
      'Screening', {
        rooms : { type: Types.Select, numeric: true, options: [{ value: 1, label: 'Screening room 1' }, { value: 2, label: 'Screening room 2' }, { value: 3, label: 'Screening room 3' }] }
  });
  </code></pre>