Added a notification system to sugar

This is largely what has been talked about on the mailing lists.This requires https://github.com/sugarlabs/sugar-toolkit-gtk3/pull/107 . Here are some pretty pictures:

![system_notifications](https://f.cloud.github.com/assets/6022042/2114017/2493d620-9036-11e3-8b82-7701882970f7.png)
System notifications can choose their icon and go on the frame

![notifications_main](https://f.cloud.github.com/assets/6022042/2114025/387f8e36-9036-11e3-9d52-7dac023306fa.png)
Notifications