Xdg config

Respect Operating System conventions regarding configuration paths.
This means ``~/.config/aws-minion`` on Linux (by default and according to [XDG Base Directory Specification](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html)) and ``~/Library/Application Support/aws-minion`` on Mac OS X.
While storing configuration directly on the home folder is recommended by [FHS](http://www.pathname.com/fhs/pub/fhs-2.3.html#HOMEUSERHOMEDIRECTORIES), I believe newer conventions are better because they don't polute the user folder.