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
rename variable childObserver to child (is now a Subscriber) and rename gps to group (is not plural)
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
Merge pull request #1304 from zsxwing/flatMap

Add flatMap and concatMap to RxScala
Merge pull request #1302 from davidmoten/groupBy-naming

improve variable names in OperatorGroupBy
0.19.0-SNAPSHOT
everything defaults to 1.6
Hooked RxJavaPlugins errorHandler up within all operators that swallow onErrors.
* Otherwise, the only onErrors visible to plugin are the ones which propagate all the way to SafeSubscriber.
testNotificationDelay - Attempt at determinism
Merge pull request #1307 from benjchristensen/make-notificationDelay-deterministic

testNotificationDelay - Attempt at determinism
Remove call to RxJavaPlugins ErrorHander in mergeDelayError operator - CompositeException is already visible