Introduce native methods for Single, Completable, Observable

[uKL]

In order to get rid of firstOrError alike calls in the client, the queue should handle operations that:

• may emit many results
• may emit one result
• may just complete

[RobLewis]

I have wondered whether Observable is the best type for establishConnection(), which can only produce either zero or one results, like a Maybe. I realize this would mean changing the "unsubscribe to disconnect" feature (which is nice), but might there be a better way?