Add function to TbActiveForm to create custom active inputs (ie: widgets)

Short example usage:

```php
<?= $form->customInputControlGroup(
    $this->widget('CustomInputWidget', array(), true),
    $model,
    $attribute
) ?>
```

Full example usage:

```php
<?php $form = $this->beginWidget('bootstrap.widgets.TbActiveForm', array(
    'id'=>'contact-form',
)); ?>

    <fieldset>

        <?= $form->textFieldControlGroup($model, 'name') ?>

        <?= $form->textFieldControlGroup($model, 'email') ?>

        <?php if (CCaptcha::checkRequirements()): ?>
            <?= $form->customInputControlGroup(
                $this->widget('CCaptcha', array(), true) . TbHtml::activeTextField($model, 'verifyCode'),
                $model,
                'verifyCode',
                array('help' => 'Please enter the letters as they are shown in the image above.<br>Letters are not case-sensitive.')
            ); ?>
        <?php endif; ?>

        <?= $form->createformActions(array(
            TbHtml::submitButton('Submit', array('color' => TbHtml::BUTTON_COLOR_PRIMARY)),
        )) ?>

    </fieldset>

<?php $this->endWidget(); ?>
```