Add support for patched wkhtmltopdf version

If you use a patched wkhtmltopdf version (archlinux https://aur.archlinux.org/packages/wkhtmltopdf-static/) for creating links. The standard options create a completely false pdf.

Sample pdf generations

Wrong

![pdf_wrong](https://cloud.githubusercontent.com/assets/722482/4225939/a8faa948-3932-11e4-89bf-7b6710f3caa3.jpg)

Right

![pdf_right](https://cloud.githubusercontent.com/assets/722482/4225949/b591dba4-3932-11e4-87b2-6255cef97dd5.jpg)