Notification timeout setting

## Overview

Be changeable settings for Desktop Notification.
Currently, setting item is **Timeout seconds until the notification close automatically**.

## Screenshots

Click **BELL** icon on the settings dialog.
![atmosphere-notification-change-icon](https://f.cloud.github.com/assets/2914192/1755255/c62fa40e-6661-11e3-8ea6-32229cf90398.png)

Input **timeout seconds**
![atmosphere-notification-setting-dialog](https://f.cloud.github.com/assets/2914192/1755257/cd60c8d4-6661-11e3-9d73-4dac512cb7d1.png)