Allow sync(callbackless) lazy attributes

It would be cool to allow callback-less functions as lazy attributes like this one:

```javascript
Factory.define('person', Person, {
  name: function(){ return 'my name'; }
});
```

This is useful when using libraries such as [Faker](https://github.com/marak/Faker.js) to define dynamic factories like this:

```javascript
Factory.define('person', Person, {
  name: Faker.Name.firstName,
  email: Faker.Internet.email
});
```

Thoughts?