Add linux version of wkhtmltopdf for Heroku

* Add wkhtmltopdf-0.9.9-static-amd64 from
    https://code.google.com/p/wkhtmltopdf/downloads/detail?name=wkhtmltopdf-0.9.9-static-amd64.tar.bz2
* Add config file to
  * Use that under staging and production
  * Use /usr/bin version running locally