WkHtmlToPdf now supports header/footer (simple)

Added helpers to facilitate easy header/footer and from View/Layout configuration of WkHtmlToPdf creation.
I think we should change up this example to make use of an element, rather than raw html dump. This will encourage users to use elements, which is far cleaner.

So
```php
$this->pdfOptions(array(
    'header-html' => $this->element('header')
));
```