Keep the paused notification open; stop or dismiss it to close it

* Keep the paused notification open so it's usable on the Lollipop lock
screen in lieu of lock screen widgets.
* Show Start, Pause, and Stop actions in the notification.
* Add a Stop button to the activity.
* Add info to the notification to explain Running/Paused/Stopped state, a
tip on setting periodic reminders, and a tip on stopping/dismissing the
paused notification.

TODO:
* Freeze the notification's timer display when paused.
* Finish reworking the activity's buttons: start, pause, reset, stop.
* Switch to the new icons. Maybe color or tint them.
* Remove the activity's Start/Stop/Reset button labels.
* Make paused reset vs. stopped look different in the activity and widget
time displays.
* Update README.md.