Sample_Code, pdf.py and PDF_Samples Contribution

1. Add isPdfValid method in PyPDF2\pdf.py
2. Add read-only properties for the creation and modification date in
the class DocumentInformation in PyPDF2\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 font size
in makesimple.py file in the Sample_Code folder