Add driver for WKHTML2PDF

[jkuchar]

Here is generic PHP Driver

https://github.com/NitMedia/wkhtml2pdf

What is actually needed to support multiple drivers:

• extract document interface which will support most common properties / use cases
• driver interface extraction (probably simple HTML in; PDF out)

[Aurielle]

Kdyby-flavored PHP driver :) https://github.com/Kdyby/Wkhtmltopdf