Fix server email and admin emails when empty
Add `autopilot` function

This is an easier way to control what Djheroku does. Just add

    from djheroku import autopilot
    autopilot(vars())

Then you can make runtime changes to your application settings:

    heroku addons:add sendgrid
    heroku config:set ADDONS=sendgrid
    heroku addons:add memcachier
    heroku config:set ADDONS=sendgrid,memcachier

Djheroku will change the variables accordingly.