Add request to get properties of a PDF file

Pretty much the same as the other two except
that it takes an additional parameter to specify
what info to retrieve.
Add javadoc to describe the different request types
Add constructer to default requestedInfo to be empty

If no info is specified then we should assume that
the user wants all of the info.
Merge pull request #3 from datalogics-tsmith/add-properties-request

Add request to get properties of a PDF file