Add ability to specify log setup script.

Summary:
With this change in place, you can specify a Python file that will be
executed when dev_appserver.py is starting up. This allows you to add
log handlers that modify the logging behavior across dev_appserver.py.

This goes with some larger changes centered around giving us more
control over logging. Most of said changes can be done within the
context of webapp's environment, but in order to give complete control,
we need a way to get at the log messages that dev_appserver.py sends
off.

Test Plan:
Add `--dev_appserver_log_setup_script="/tmp/myscript.py"` to the
`dev_args` variable in webapp's Makefile. I then observe that the test
script is able to attach logging handlers that stick around. No errors
occur if the argument is not provided.

Reviewers: dylan

Reviewed By: dylan

Subscribers: dylan

Differential Revision: http://phabricator.khanacademy.org/D14222