pdfcat et al.: now 3.0-compatible.

modified:   PyPDF2/pagerange.py
    Define "Str" (capital S) to mean basestring in 2.x, str in 3.x.
    Used while validating a PageRange initializer.

modified:   Sample_Code/makesimple.py
    from __future__ import print_function
    I guess this would work, except makesimple.py uses reportlab, which
    isn't 3.x compatible yet.

modified:   Sample_Code/pdfcat
    from __future__ import print_function
    Aargh!  PdfFileMerger in 3.x requires that the input files stay
    open while it's doing the write().    So, I went back to caching all
    the  open files in pdfcat.  I do not close them, since pdfcat is just
    a script and exits immediately.  But at least each file is only open
    with one file object.
    Tested in 2.7 and 3.3.  Redirecting stdout works in both on Mac OS X.