Adding the possiblity to define engine options.

That way we can set any options the engine may have, or override any hardcoded/default options.
i.e: http://madalgo.au.dk/~jakobt/wkhtmltoxdoc/wkhtmltopdf-0.9.9-doc.html
Creating a mocked binary of wkhtmltopdf to run on Travis.
Merge pull request #32 from renansaddam/feature/WkHtmlToPdfOptions

Adding the possiblity to define engine options.