Added ongoing option (android only)

Set value of smallIcon to icon by default as it was in the past before the smallIcon could be defined yourself.
Reported by #59

If the trigger date was in the past, the notification will be displayed immediately.

Fix missing Date import in Restore.java

Always restore Android notifications with a minimum date of now

This change prevents missing a notification that was scheduled to fire when the
device was off by re-registering it for right now. This matches the behaviour on
iOS.

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.