Complete description of Python web server in terms of extensibility.

Rewrote much of the code and the second half of the chapter in order to focus on extensibility.
PEP257: For consistency, always use """triple double quotes""" around docstrings.

I know single triple quotes are valid, too, but I think it's important to stick to the conventional and more popular triple-double-quotes in a book that's aimed at junior-level programmers.