Made tail go until "shutdown" keyword is encountered.

The subscribeOn and observeOn calls do not work properly -- they make tail
terminate immediately without reading anything. Or perhaps the problem is the
`while (!Thread.interrupted())` call. I am not sure.

The bigger question: how do I shut down the observable when the observer is done
with it? The observer knows to stop when it sees the `shutdown` keyword; must I
program that into the observable as well?