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.
It would probably be best if we treated logging the same, as much as possible, between environments (12factor style!) 

Additionally, it occurred to me that we may not be logging stack traces?