Prepare Method for PDF

As discussed on the [mailing list](http://www.openwall.com/lists/john-dev/2013/01/24/19) I added a prepare method to the pdf_fmt_plug.c.  

The prepare method added to pdf_fmt_plug.c does the following:
* Checks if the format is the old style of pdf format. If it isn't return what prepare normally would otherwise:
* Check if input in the old format is valid. If it isn't return an invalid string otherwise:
* Converts the input to the new format and returns it.

I presumed this was needed for jumbo-8 but if it isn't sorry feel free to reject it and I'll send another pull request to the bleeding branch.