Makefile: only make the `part*.tex` files.

I.e., omit `TEMPLATE.tex` etc.
Update slides for the Sept. 11--12, 2013 course.
refs.mdwn: Add references to PythonChallenge.com and the Python Koans.
Revise and update `part00`.

* Introduce the GC3 on the first slide
* Fix URLs
Revise and update `part02`.

* Rename function `hello.hello` to `hello.greet`
* Add calc exercise (compute 2**38)