More consistent hooks for scheduler plugins.  This is a rework of #1514.
* Deprecated RxJavaDefaultSchedulers and RxJavaDefaultSchedulersDefault (I think these names are confusing)
* Added RxJavaSchedulersHook
** Ability to override entire schedulers from scratch
** Ability to decorate Action0 before submission to Scheduler
Merge pull request #1559 from mattrjacobs/scheduler-plugin

More consistent hooks for scheduler plugins.  This is a rework of #1514.