comply with XDG Base Directory Specification (fixes #13)

User config tree now defaults to ~/.config/rapport, and
reports dir to ~/.local/share/rapport/reports.  Added a
new unit test for the latter.

http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
Merge pull request #14 from aspiers/xdg-compliance

Let's have it merged then. I'll add another commit to keep the old config locations with lower priority. If necessary, migration code can still be written later on