Migrate exercise-screens-daemon to an upstart job.

Summary:
Migration Plan:
sudo /etc/init.d/exercise-screens-daemon stop
sudo update-rc.d -f exercise-screens-daemon remove
cd ~/aws-config
git pull
cd ~
sudo ln -sfnv ~/aws-config/etc/init/exercise-screens-daemon /etc/init
sudo start exercise-screens-daemon

Test Plan:
Ran it locally in a VM, with a different S3 account and bucket.

Killed the python process; watched upstart restart it.

Viewed the logs in /var/log/upstart/exercise-screens-daemon.log to make sure
nothing strange was happening.

Reviewers: cbhl

Reviewed By: dylan

CC: stephanie, alpert

Differential Revision: http://phabricator.khanacademy.org/D2052