Fix running error due to PyPDF2 upgrade

[panando]

Some methods in PyPDF2 （version > 1.2.8）library have deprecated, the current version of pdfsm will raise a running error with PyPDF2 > 1.2.8.
I changed the codes to make it runable with PyPDF2 > 1.2.8.
pdf_split_merge.py was modified.

PdfFileWriter --> PdfWriter
PdfFileReader --> PdfReader
pdf.getPage(p) --> pdf.pages[p]
pdf.addPage(p) --> pdf.add_page(p)
pdf.getNumPages()--> len(pdf.pages)

https://github.com/panando/Pdf-Split-Merge/blob/master/pdfsm/pdf_split_merge.py