Added Python 3 support for pdf2john

pdf2john.py should now work with Python 3 as well as Python 2. Tested it against all the file from the wiki and it worked fine on:
* Python 3.2.3
* Python 2.7.3


Dhiru @kholia asked me to do this earlier today:
https://github.com/magnumripper/JohnTheRipper/issues/294