Notification Improvements!

Replaces #393

Changed based on @tchx84's comments:
![Imgur](http://i.imgur.com/WGabUCg.png?1)

Feature pages:
* [Urgency](http://wiki.sugarlabs.org/go/Features/Notification_Urgency)
* [Replacement](http://wiki.sugarlabs.org/go/Features/Notification_Replacement)
* [Progress Bar](http://wiki.sugarlabs.org/go/Features/Notification_Progress_Bar)