Improvement of repeating notifications at exact time (#44). `AlarmManager.setRepeating` triggers the notifications at strange dates, like everything others with Android...

Fix to callback function calls

fix to callback function call, removed deprecated call

Add `autoCancel` flag to indicate that the notification shall be automatically canceled when the user clicks it

App throws an error on iOS if `message` is null. - Removed extra line break on iOS if `title` is null or empty.

Images or sounds must be placed into the res-folder and not into the assets-folder.