Implement Observable.takeUntil()

For my project I needed ``Observable.takeUntil()``. As I have no prior experience with the monifu.js codebase, I wonder how to implement new operations. In particular, I noticed that some methods are using locking, but not all of them. When is this required?

Also, I suspect that my use of ``tick()`` in the test case is not entirely correct. I would like to write a test case which pushes a sequence of elements, but at a certain element a second Observable pops in, pushing one element which then completes the Observable returned by ``takeUntil()``. This will ensure the correct behaviour when the processing was succesful. We'd also need a test case checking the error behaviour. As this is not done for the other operations either, is there a best practice?

By the way, IntelliJ stripped some trailing whitespaces. I hope that's not a problem.
You do not need to take an `implicit scheduler` as parameter. The scheduler is already part of the `Observable` (in comparison with `Future`). This is because the `Observable` has a different design - we could talk about it because I'm not sure if I did the right thing, but for now this parameter doesn't belong here.