Remember working state across restarts

 - Fix setting the initial value by using pomodoro.left with `settime`.
 - Use `math.min` to get the lower value for `pomodoro.left`, in case
   the config/setting has been changed.
 - Move call to `awful.tooltip()` to the end of `init`.
 - Fixes https://github.com/nikolavp/awesome-pomodoro/issues/15.