Add button, fieldset support.

I've added button and fieldset support:

```php 
$fom = new WP_Form( 'form-name', '', arary( 'id' => 'form-name' ));

// Fieldsets are useful when complex forms need various sections to be separated.
// Store them in a variable and then add inputs just like with a WP_Form:
$fieldset1 = $form->fieldset( array( 'class' => 'fieldset-one' ));
    $fieldset1->input('name', array('label' => 'Company Name'));
    $fieldset1->email('email', array('label' => 'Preferred E-Mail'));

// Buttons are good when html markup wouldn't work in the value attribute of an input
$form->button('submit', array(
  'type' => 'submit',
  'class' => 'submit-btn',
  'value' => 'submit',
  'label' => '<i class="fa-check"></i> Submit'
  ));

echo $form->build();
```