Refactor Subscriber to support backpressure in lift
tail backpressure support
toXXX backpressure support
Remove a Subscriber.apply overload and update the examples

1. Remove Subscriber.apply[T](subscriber: Subscriber[_], onNext: T => Unit, onError: Throwable => Unit, onCompleted: () => Unit)
2. Update the examples
docs for Subscriber
Merge pull request #1 from samuelgruetter/subscriber-doc

docs for Subscriber
Add comments and a unit test for `apply(rx.Subscriber)`; change the visibility of `request` to `protected[this]`
Merge pull request #57 from zsxwing/subscriber

Refactor Subscriber to support backpressure when using lift and bug fixes