Fix NumberFormatException in AlarmReceiver.

+ Application crashed when alarm was due for a lecture. Bundle keys for
  add and delete alarm actions must be identical.
+ Bug was introduced here: 62e2890750d36b8988440d496b06d632590d6551
Merge pull request #33 from johnjohndoe/bugfix/alarmreceiver-numberformatexception

Fix NumberFormatException in AlarmReceiver.