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 do like how it reads now. My only problem is that you don't get an actual `Queue` instance back which is a bit misleading. Type safety would save you, but I totally would have written `Queue foo = Queue.of(..)` on my first use.