Rework config and data directories lookup

Adhere to XDG Base Directory specification on Linux, use `%APPDATA%` on
Windows and `~/Library/Application Support` on OS X as base relative
directories for data and config files.
Fix directory creation
Force create config directory
Update README.md
Small fix
Merge pull request #119 from smaximov/xdg-basedir-spec

Rework config and data directories lookup