PDF Metadata values grabbed by pdf2john

pdf2john now grabs certain values from the pdfs xmp metadata if the metadata is left unencrypted. At the moment it grabs:

* Title
* Creator/Author
* Subject 
* Tags/Description
* Year of creation

##### Pull request also includes:

* A bug fix for how objects in pdf files are retrieved by pdf2john. This fix probably should be put into unstable.
* A fix for checking if metadata in encrypted in python 3. It previously broke if metadata wasn't encrypted. I've included a sample file on the wiki so something this doesn't happen again.