idPrefix is always `undefined_`

`render` method of `Form.editors.NestedModel` defined `this.nestedForm` as follows:

```javascript
  render: function() {
    //Get the constructor for creating the nested form; i.e. the same constructor as used by the parent form
    var NestedForm = this.form.constructor;

    ...

    //Wrap the data in a model if it isn't already a model instance
    var modelInstance = (data.constructor === nestedModel) ? data : new nestedModel(data);

    this.nestedForm = new NestedForm({
      model: modelInstance,
      idPrefix: this.id + '_',
      fieldTemplate: 'nestedField'
    });
```

Thus, `idPrefix` attribute for `NestedForm` options hash was always `undefined_`.

I changed the line `idPrefix: this.id + ' '` to `idPrefix: modelInstance.cid` in order to have unique field ids.

I also built for distribution.

I think it is now correct. If it's not, please feel free to reject my pull request: you won't hurt my feelings :)