ReactiveX/RxScala — pull request #57 by zsxwing (2014-11-08 16:51:02+00:00)
Title: Refactor Subscriber to support backpressure when using lift and bug fixes

I found `tail` and `toXXX` didn't work after adding backpressure. To solve it, I have to change `Subscriber` to `abstract class` so that `Subscriber`s can be chained and call `request` properly.