Reimplement 'subscribeOn' using 'lift'

hi, this PR reimplemented the 'subscribeOn' using 'lift'. However, both the original and current implementation can not guarantee that 'unsubscribe' is always called in the `scheduler`. A extreme example is:

```java
    public static void main(String[] args) throws InterruptedException {
        Observable.create(new OnSubscribe<Integer>() {

            @Override
            public void call(final Subscriber<? super Integer> t1) {
                final Subscription s = Subscriptions.create(new Action0() {

                    @Override
                    public void call() {
                        System.out.println(Thread.currentThread().getName());
                    }
                });
                t1.add(s);
                new Thread(new Runnable() {

                    @Override
                    public void run() {
                        t1.unsubscribe();
                    }

                }, "test").start();
                try {
                    Thread.sleep(10000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                t1.onCompleted();
            }
        }).subscribeOn(Schedulers.newThread()).subscribe();
        Thread.sleep(10000);
    }
```
will output "test".