WkHtmlToPdf now supports header/footer (simple)

Added helpers to facilitate easy header/footer and from View/Layout configuration of WkHtmlToPdf creation.
I think it would be better to use a nested array here:
```php
array(
    'header' => array(
        'spacing' => 8,
        'html' => 'file.html',
        'left' => $title_for_layout
    )
)
```