Contribution to Sample_Code, PyPDF2 and Samples

1.Add a isPdfValid method at PyPDF2\pdf.py
2.Add read-only properties for the creation and modification date in the
class DocumentInformation at pdf.py.
3.Add 2-up-merging.py file in the Sample_Code folder
4.Add pdf_info.py file in the Sample_Code folder
5.Add 3 pdf files in the PDF_Samples folder
6.Offer the choice to the user to select the font family and fond size
in makesimple.py file in the Sample_Code folder