fixing inline form validation display for Zurb

This changes the inline help text for form validation from

```
    <small>help text</small>
```
to
````
    <span>help text</span>
````
This allows attributes to be set correctly without working around the ```<small>``` tag, like in the TwitterBootstrap adapter, for example:

````
Former::text('somefield')
        ->label('Some Field')
        ->state('error')->help('Some field is required.', [ 'class' => 'alert-box warning radius'] )
```
I didn't add a test - let me know if you need one for this.