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
I believe this is actually an artifact from a time where we didn't have the interface.

This is also re your comment about this class not being final. We used to mock this in unit tests which was a terrible idea. I'll make it final now that we have `TestEventBus`.