rename gunicorn.py to avoid import errors

I was playing with the gunicorn branch and I ran into this:
```
Traceback (most recent call last):
  File "/usr/local/bin/gunicorn", line 7, in <module>
    from gunicorn.app.wsgiapp import run
ImportError: No module named app.wsgiapp
```
I had `ENV PYTHONPATH /app` in my Dockerfile so that I could run manage.py from another location, except then the gunicorn.py configuration file got picked up instead of the actual gunicorn package.

Renaming the configuration to `gunicorn_conf.py` fixed the problem for me.