Adding a thumbnail method

[ntaylor-86]

Adding the ability to create thumbnails from a pdf.

$pdf->setThumbnailWidth(400)
    ->saveImage($pathToWhereImageShouldBeStored);

I was looking for this feature in this package but didn't find it.
Hopefully others will find this helpful as well.

Cheers, Nathan.

[freekmurze]

Thanks!