Enable Flask's app.logger under Gunicorn

[hkeeler]

Flask's built-in logger does not work by default when run through Gunicorn. See the following for potential fixes:

• Log messages from Flask's app.logger benoitc/gunicorn#379 (comment)
• http://stackoverflow.com/questions/24501444/how-to-see-exceptions-in-a-flask-gunicorn-app/24508613#24508613
• http://stackoverflow.com/questions/28982106/flask-app-logger-not-working-when-running-within-gunicorn