Feature: Sent notification listener

<code>PushManager</code> can now notify your code when specific notification is considered as "sent". I think that this good companion to <code>RejectedNotificationListener</code>.

Implementation is not perfect (<code>SentNotificationListener</code>s are not fired until notification is removed from sent buffer), but i am open for any ideas for improvement.

Note, that this pull request assumes that pull request #82 was not merged.