fix concurrency bug in ScheduledObserver

- found a concurrency bug while working on https://github.com/Netflix/Hystrix/issues/123
- the following code would lock up occasionally due to onCompleted not being delivered:

```java
public class RunTest {
    public static void main(String[] args) {
        System.out.println("Starting test...");

        final ArrayList<String> strings = new ArrayList<String>(200000);

        int num = 10000;
        while (true) {
            long start = System.currentTimeMillis();
            final AtomicInteger count = new AtomicInteger();
            for (int i = 0; i < num; i++) {
                new TestService1(2, 5).toObservable().forEach(new Action1<Integer>() {

                    @Override
                    public void call(Integer v) {
                        count.addAndGet(v);
                    }
                });

                new TestService2("hello").toObservable().forEach(new Action1<String>() {

                    @Override
                    public void call(String v) {
                        strings.add(v);
                    }

                });
            }
            long time = (System.currentTimeMillis() - start);
            long executions = num * 2;
            System.out.println("Time: " + time + "ms for " + executions + " executions (" + (time * 1000) / executions + " microseconds)");
            System.out.println("   Count: " + count);
            System.out.println("   Strings: " + strings.size());
            strings.clear();
        }
    }
}
```

- Also made OperationObserveOn not use ScheduledObserver if the `ImmediateScheduler` is chosen to allow an optimization. I believe this optimization is safe because ScheduledObserver does not require knowledge of a Scheduler (such as for now()) and all we do is emit data to the Observer on a scheduler and if we know it's Immediate we can go direct and skip the enqueuing step. This allows shaving off a noticable number of microseconds per execution in the loop above.
Merge pull request #268 from benjchristensen/observeOn-fixes

Fix concurrency bug in ScheduledObserver
Gradle Release Plugin - new version commit:  'rxjava-0.8.4'.
Fix concurrency bug in ScheduledObserver

This is a followup to https://github.com/Netflix/RxJava/commit/1fa6ae3be23200c787cc1c25c0bb8cac99ae0d17 that fixed one issue (concurrency) and created another (broke Rx contract by allowing concurrent execution of onNext).

I have reverted back to the previous implementatio and then attempted to fix the concurrency issue again.

I think it ended up being a simple fix … just re-ordering the `enqueue` method to remove the race-condition between the logic protected by the AtomicInteger and adding to the queue.

It's not an atomic operation (adding then processing) so we need to just add to the queue and treat it as an async data structure and keep the AtomicInteger portion to only protecting the "process or not process" logic.

```java
        // this must happen before 'counter' is used to provide synchronization between threads
        queue.offer(notification);
```

This may still have issues but it's now working in all of my concurrency tests (the ones that broken with the original and then my modified version). The tests are not easy to build unit tests out of as they require running for many seconds and non-deterministically causing a race condition so I have not yet spend the time to try and figure out a deterministic unit test hence them not being committed.
Merge pull request #269 from benjchristensen/observeOn

Fix concurrency bug in ScheduledObserver
Gradle Release Plugin - new version commit:  'rxjava-0.8.5'.