Cross-reference observeOn and subscribeOn from each others' javadocs
CompositeException does not mutate nested exceptions by attaching stacks, but instead
aggregates them at print-time.

Since nothing is being mutated, there's no chance of accidentally creating a cycle in the Exception chain.
Removed unused import
Fixed up SafeObserverTest w.r.t. CompositeException changes
Merge pull request #1388 from mattrjacobs/eliminate-circular-exception-creation

CompositeException stops mutating nested Exceptions
rxjava-scalaz

commenting out rxjava-scalaz until I can figure out what's wrong with the Netflix building system that prevents it from finding the following jar:

```
Artifact 'org.scalaz:scalaz-core_2.10:7.0.4@jar' not found.
```
[Gradle Release Plugin] - pre tag commit:  '0.19.2'.
[Gradle Release Plugin] - new version commit:  '0.19.3-SNAPSHOT'.
Version 0.19.2
Backpressure - Merge & ObserveOn
Performance by eliminating SubscriptionList/Subscriber & Synchronization

This is an exploratory performance improvement. See discussion at https://github.com/Netflix/RxJava/pull/1384
RuntimeException if invoking request when it can't be
Fast-path for Range

53m -> 160m messages/second when consuming an Observable.range(0, 1000000)
Improve Range/Iterable Perf on Backpressure

Avoid touching a volatile inside the tight loops.
The OperatorObserveOnPerf.observeOnComputation with 1000000 values was taking so long to run I killed it. With these changes it completes:

r.o.OperatorObserveOnPerf.observeOnComputation   1000000  thrpt         5        7.613        6.672    ops/s