Keep another variable in xrdb for started state

This should fix #15. It keeps another variable(number) in xrdb to
indicate if the timer was started or stopped when awesome restarted.
Trim trailing whitespace
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.
Merge pull request #17 from blueyed/keep-working-state

Remember working state across restarts (based on started state branch)