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
It's a good point. I used to be called `QueueDescriptor` because `Queue` is also confusing in the sense that it's not the actual queue, it's a token if you will, a descriptor type used to capture identity and type information.

I could live with `EventQueue`. I had thought about `Channel` too, since on top of what you mentioned, the fact that it uses `HandlerThreadScheduler` by default (and therefore Android's message queues) is actually an implementation detail and could be changed. So technically these are not queues, more data channels/pipes.