Add support for non-inline Multicheckbox element

Ok, this PR allows to create MultiCheckbox elements in non-inline display.

Adding an option to the element

```php
$form->add(array(
    'name' => 'multiCheckboxElement',
    'type' => 'MultiCheckbox',
    'options' => array(
        'value_options' => array(
            array('label' => '1','value' => 'option1', 'attributes' => array('id' => 'noInlineCheckbox1')),
            array('label' => '2','value' => 'option2', 'attributes' => array('id' => 'noInlineCheckbox2')),
            array('label' => '3','value' => 'option3', 'attributes' => array('id' => 'noInlineCheckbox3'))
        ),
        'inline' => false
    )
));
```

The **inline** option is under 'options' as it doesn't affect the element, and allows to enable/disable the Module without further modifications.