Logging using a dedicated logger

overviewer was logging everything to the root logger. This is not recommended. This merge will make overviewer use two dedicated loggers (because `overviewer.py` does not live in the same package as `overviewer_core`)!

Additionally, the logging configuration code (logger.configure) has been simplified, and will route to both `stdout` and `stderr` (depending on log record level).

We could go from two loggers to only one by simply renaming the logger in `overviewer.py` to `overviewer_core`!