Notes on issues 13 and 23

- #13
Added Pomodori::Event#done_today class method

- #13
- Spec test pending
Making Pomodori::Event#determine_kind a class method

- #13
Adding tests around done_today

- #13
- Adjusting determine_kind test, too
Implements #done_today for Pomodori::Events

- Fixes #24
- Fixes #13
- Also implements database cleaning on test runs (and adjusts spec tests to
  handle that)