updated wkhtmltopdf and use patched qt

wkhtmltopdf was very old. Also, compiling against stock version of qt4
disables a lot of features:
  * Running without an X11 server
  * Printing more then one HTML document into a PDF file
  * Adding a document outline to the PDF file
  * Adding headers and footers to the PDF file
  * Generating a table of contents
  * Adding links in the generated PDF file
  * Printing using the screen media-type
  * Disabling the smart shrink feature of webkit

I haven't set the meta.platform right now. What is nixpkgs policy there? It would
be convenient to have binaries since the build takes a long time because of qt
but it might not be a very popular package.