2 new tests for OnSubscribeToObservableFuture

1) Creating an Observable from an already-cancelled Future should emit nothing (equivalent to Observable.never())
   (it currently emits onError(CancellationException)
2) Creating an Observable from a Future then unsubscribeing immediately before Future.get()
   unblocks should be an Observable.never().  This test already passes.
Modify OnSubscribeToObservableFuture to return early if unsubscribe without emitting anything
add annotations about default schedulers to javadocs for Observable operators
Fix and Update JMH Perf Tests

- The tests were wrong and re-using a single Subscriber instance which meant they weren't really testing much.
- Same with the CountDownLatch which meant they weren't waiting if async.
- Added several SerializePerf and PerfTransform tests
Merge pull request #1293 from benjchristensen/perf-tests

Fix and Update JMH Perf Tests
Merge pull request #1291 from mattrjacobs/check-unsubscribe-within-observable-from-future

Check unsubscribe within observable from future
rx.operators -> rx.internal.operators
Merge pull request #1294 from benjchristensen/rx-internal-operators

rx.operators -> rx.internal.operators
hard-code image heights in javadocs to make page loading cleaner, especially with links to anchors deep in the javadoc
Change `void accept` to `boolean accept`

As per a change @akarnokd did elsewhere with `accept2` method.
Merge pull request #1295 from benjchristensen/notification-lite

Change `void accept` to `boolean accept`
Move RxThreadFactory out from NewThreadScheduler
Move NewThreadWorker out from NewThreadScheduler

- Break out ScheduledAction from NewThreadWorker
Merge pull request #1296 from jbripley/schedulers-internal

Move re-used internal Scheduler classes to their own package
Remove Bad Perf Test

- this just stresses scheduling and at this throughput creates massive garbage and tests the wrong thing
Merge pull request #1298 from benjchristensen/perf-transform

Remove Bad Perf Test
Add convenience method for adding unsubscription callback
Merge pull request #1301 from jbripley/subscription-block-expression

RxScala: Add convenience method for adding unsubscription callback
Add flatMap variants to RxScala
Add concatMap to RxScala