observeForever is not implemented

[matsujun]

    @MainThread
    override fun observeForever(observer: Observer<in T>) {
        val wrapper = ObserverWrapper(observer)
        observers.add(wrapper)
        super.observeForever(wrapper)
    }

is not implemented.

So LiveEvent is not possible to use source of MediatorLiveData.
(MediatorLiveData uses observeForever)

[hadilq]

Thanks for your contribution. Merge the #22 PR to solve the problem.