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.
Special request - the timeout should be a `Duration` instead of a `FiniteDuration`. This would allow us to specify `Duration.Inf` instead of a finite duration with the behavior that everything would get buffered (until the end of time) and a Seq would be emitted on completion.

Of course, we can debate this. Do you think a `Duration.Inf` would be useful?