Concat Backpressure

Route backpressure through the inner subscribers.
Fixes https://github.com/Netflix/RxJava/issues/1480
Amb with backpressure support
Propagate request(n) to each AmbSubscriber

... and then keep propagating to the winner.
Merge pull request #1533 from benjchristensen/issue-1516

Amb + Backpressure
Merge pull request #1534 from benjchristensen/concat-backpressure

Concat Backpressure
[Gradle Release Plugin] - pre tag commit:  '0.20.0-RC4'.
[Gradle Release Plugin] - new version commit:  '0.20.0-RC5-SNAPSHOT'.
Version 0.20.0-RC4
add "@since 0.20" annotation to new multicast() variant
Fixing a bug and a potential for other concurrency issues.
RxScala: Add retryWhen/repeatWhen methods

- Add example usage of retryWhen/repeatWhen to RxScalaDemo
Merge pull request #1555 from jbripley/rxscala-retrywhen

RxScala: Add retryWhen/repeatWhen methods
Merge pull request #1552 from abersnaze/most-recent-null

Fixing a bug and a potential for other concurrency issues.