do not stop timer when the phone gets locked.

[fespinoza]

related to https://github.com/fespinoza/Todoro/commit/c12d025c05ac931af50cc8809cd32aa9ba1ffa02

TODO:

• [x] restore the correct time when the app goes to foreground after being in background.
• [x] show local notification when the pomodoro time passes and the app is in the background.
• [x] execute the pomodoro completion code when the local notification is triggered.
• [x] show alert when trying to navigate back and a pomodo is running (cancel pomodoro if confirmed)
• [x] make sure there are no bugs regarding pomodoro presence.
• [x] make alert different for breaks and for pomodoros.

[fespinoza]

relevant: https://stackoverflow.com/questions/27585265/can-a-local-notification-trigger-code-in-ios8