Remove groupBy with selector.

Use groupBy.map instead.
Deprecate Direct Access to Scheduler Implementations

This enforces the convention of using `Schedulers.*` and then makes the RxJavaDefaultSchedulers plugin more reliable.
Scheduler instances should be or behave like singletons

- remove the Func0 factory signature for Scheduler creation.
Merge pull request #909 from benjchristensen/scheduler-plugin

Scheduler Plugin Refactor
Merge pull request #910 from benjchristensen/groupBy-selector

Remove groupBy with selector.
[Gradle Release Plugin] - pre tag commit:  '0.17.0-RC3'.
[Gradle Release Plugin] - new version commit:  '0.17.0-RC4-SNAPSHOT'.