using context manager in files tutorial
migrated Leszek's changes to strings-io/tutorial.ipynb
merging in Leszek's changes to gh-pages

Thanks to @czterybity for this contribution, and to @wking for
mentoring this pull request.

Thanks to @r-gaia-cs, @mattphotonman, @gvwilson, @wltrimbl, and
 @ethanwhite for helpful commentary and feedback.