Add autoclosing feature for browser tabs


![pr](https://cloud.githubusercontent.com/assets/2142829/5423690/9698497c-82ca-11e4-9b1a-39a24285c099.gif)
Close the tab on `kill`.
`kill` is triggered when the livedown server shuts down.
Hey @shime.
Can you please explain the difference to me when using `window.open('', '_self', '');`.
In my understanding `window.close()` will work in our case without the warning.
It only throws the message when trying to run the command not in an included script.