Add a config setting to log to a file

This logs all exception & stack traces by default.

If you want to log a warning, you can do something like:

```python
app.logger.warn("Something is bad!")
```

and it will show up in either the log file or the console depending on settings.
I pushed a change which does away with DEBUG entirely, in favor of the more precise USE_RELOADER. This makes exceptions and `app.logger` messages get logged identically in dev & prod.

Individual requests are logged by werkzeug, so you only see them when you're running locally. We can try to turn those on in production if you feel strongly about it.