Add options to add extra CSS classes

Hello,

These two new options allow to add custom CSS classes to labels and wrappers around fields. We have now more control on forms style.

Example :
```javascript
var aForm = forms.create({
  name: fields.string({
    required: true,
    label: 'fieldname',
    extraClasses: ['an-extra-wrapper-class', 'another-extra-wrapper-class'],
    labelClasses: ['an-extra-label-class']
  })
});
```

Thanks for your feedback.