Refactored __init__.py and unified logging.

- Use common python patterns to determine BASE_PATH
- Use xdg_data for logfile and whitelist rather than xdg_home
- Renamed log to caffeine.log.
  see also: https://wiki.debian.org/XDGBaseDirectorySpecification#state
- Unified logging:
  - Create handlers and formatter in __init__.py
  - Use logging.getLogger(__name__) in a module

This tool needs **a lot** more refactoring like this.
The code is still quite confusing.