Proposal: Log to stderr by default. #832

My proposal for issue #832. I realize it's a change from version 19 and may still need more discussion.

This also removes all print statements from the `gunicorn/` directory.
I'd prefer

```py
from __future__ import print_function

print(msg, file=sys.stderr)
```

Also, it would be good if you could open a new PR for the stderr changes. Thanks! :)