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're not respecting the [back-pressure requirement](https://github.com/monifu/monifu/wiki/Rx-Contract-and-Design-Guidelines#141-must-only-send-onnext-events-in-response-to-demand-signaled-by-the-observer) here. Since you've got a complete sequence in there, I recommend doing it like this:

```scala
Observable.from(buffer).unsafeSubscribe(observer)
```

`Observable.from` takes a sequence, pipes it to the observable and does respect the contract.