RPC Interface: Add get_server_status() method - Dashboard

Add a new get_server_status method so we can get the overall status of the autotest server through the RPC interface, so that we can display to the users when any of the services is malfunctioning. I've implemented a rough proof of concept of the UI in the web interface prototype I'm working in, see screenshot.

![screenshot from 2013-09-11 20 22 12](https://f.cloud.github.com/assets/296807/1127358/15cc03f0-1b39-11e3-85d0-687e62524cda.png)


![screenshot from 2013-09-11 20 23 52](https://f.cloud.github.com/assets/296807/1127361/52083e1a-1b39-11e3-862a-96c7e9c63b67.png)