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
Also, +`final`. I'm a big fan of `final`-all-the-things and non-`public`-all-the-things unless there's concrete reason to do so otherwise.