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.
I'm weakly inclined to agree with @dlanger. Executing separate programs to handle requests is more infrequent today, with WSGI modules for Apache and other web server-based handlers that optimize execution overhead away. It might be better to mention that executing an external program is one way to extend the content a web server can serve.