adding a simulation mode

[Devin Jeanpierre]

For what it's worth, this is the reason that Allen Short wrote Exocet.

> This way, you can test your code without having to resort to sys.modules
> hackery, and you can better factor your applications by separating
> configuration and environment concerns from the rest of your code.

See:

- http://washort.twistedmatrix.com/2011/01/introducing-exocet.html
- http://washort.twistedmatrix.com/2011/03/exocet-second-look.html

-- Devin