Respect XDG Configuration Spec

[alichtman]

Currently, psensor creates $HOME/.psensor/ to store configuration data and logs. This creates some clutter in $HOME that could be organized nicely.

• Configuration should be under ~/.config/psensor/.
• Cache should be under ~/.cache/psensor/ (e.g. logs).

See:

• http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
• http://ploum.net/207-modify-your-application-to-use-xdg-folders