Mention IPython notebook on key differences between Py2 and Py3.

Fix Issue #6.
Reference S3IT instead of GC3 in the text.
Add link to R. Kettler's writeup on "magic" methods.

Fix GitHub Issue #1.
More on Py2 vs Py3
Link to L. Giordani's "OOP concepts in Python 2.x".

Replace mention of "Python for Java programmers",
which does (no longer) fit our typical audience.

Fix GitHub Issue #4.