Notification improvements

Notification improvements (continued from #372)
- use NotificationCompat.Builder from the support library and remove the custom progressbar layout; this will give platform users the expected look and feel of a notificati
- remove OCNotificationManager (it contained the old layout references, but was unused)
- add new notification icons that follow the Android style guidelines (http://developer.android.com/design/style/iconography.html#notification)

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