Subject based event bus (rxandroid-eventbus)

This is a long standing thing I meant to pull out of our app which has proven invaluable for us. It's just a thin wrapper around Rx subjects, but with an API that lends itself well for dependency injection.

Sample activity:
https://github.com/ReactiveX/RxAndroid/pull/60/files#diff-18

Sample service:
https://github.com/ReactiveX/RxAndroid/pull/60/files#diff-19
If you mean `onCompleted` won't fire, yes that's exactly true and intentional, as it doesn't make sense for an event queue to complete. It must stay open. That is not violating the Rx contract, any terminal events are optional. (See `Observable.never`)

The differences to Otto? It's based on Rx. :-) So you don't have to juggle two different event systems, and you can do something like this:

```
Observable.just(1, 2, 3).map((i) -> new MyEvent(i)).subscribe(EventQueue.MY_EVENT)
```

i.e. you can take any sequence you already have, map it to an event type you define and publish it on the event bus. I guess you could write Otto adapters to accomplish that, but in general Rx is a super set of the general concept of event buses, so that would strike me as an odd thing to do.