Display a status icon on the Network status menu entry

Live preview: http://bitcointest2.us.to/en/

Seems convenient to me and a good way to draw more attention on the status of the network.

![capture du 2014-10-28 00 09 17](https://cloud.githubusercontent.com/assets/3578089/4803602/8ab4f752-5e5a-11e4-9f47-ef9ac1bdff38.png)