Add support for gunicorn

Adds gunicorn with eventlet workers.

I added support for [eventlets](http://gunicorn-docs.readthedocs.org/en/latest/design.html#async-workers). I'm not right sure if we still have to do something like [setting an amount of workers](http://gunicorn-docs.readthedocs.org/en/latest/design.html#how-many-workers) but I thought this is not necessary for the eventlet worker. For further information see [here](http://eventlet.net/) especially this line seems to indicate we do not need to set a amount of worker (manually):
```"pool = eventlet.GreenPool()" constructs a GreenPool of a thousand green threads.```([source](http://eventlet.net/doc/design_patterns.html#client-pattern))
(Gunicorn is probably doing this or something similiar internally)

Was tested on heroku.

`--log-file -` (note the `-` at the end) will post any logs to stdout, which you can obtain from heroku via `heroku logs` if you installed the heroku toolbelt.