When setting up virtualenv, set up a .dbg virtualenv as well.

Summary:
I even create a README.debugging to tell people how to use it.  This
has proven useful to me in the past, and rather than having to set it
up each time, it would be nice if it were part of the standard setup.

Test Plan: Fingers crossed.

Reviewers: chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D13575