Add user notifications reporter which sends notifications to NSUserNotificaitonCenter after an action succeeded or failed.

Examples of notifications:
https://dl.dropboxusercontent.com/u/6872784/tmp/xctool/xctool_notification.png
https://dl.dropboxusercontent.com/u/6872784/tmp/xctool/xctool_notifiction_center.png
Merge pull request #346 from facebook/nekto/add_notifications

Add user notifications reporter which sends notifications to NSUserNotificationCenter after an action succeeded or failed.