Add example code listed in the book's Web page to this repository

[ramalho]

The book's Web page provides links to the source files listed below. None of them is in this repo, but they should be. There is a coke.py file in this repo at code/sync_code/coke.py but it is not the same coke.py linked from the book's page.

• threading_cleanup.py
• counter.py
• counter_mutex.py
• coke.py
• counter.c
• counter_mutex.c
• semaphore.c

I'm not sending a pull request to add the files because I'm not sure where you'd like to put them.

I opened this issue because I wanted to make a pull request fixing threading_cleanup.py to run on Python 3, but I couldn't because that file is not in this repo.