Make RACQueueScheduler initializer protected

RACQueueScheduler should not be instantiated directly, because it
becomes too easy to violate the serialization contract of RACScheduler.
Add `name` parameter back to RACQueueScheduler
Documentation improvements to RACTargetQueueScheduler
Use a default name when nil for RACTargetQueueScheduler
Use the real class name in the default RACScheduler name
Fix up RACTestExampleScheduler interface
Merge pull request #562 from ReactiveCocoa/more-queue-scheduler-changes

Further RACQueueScheduler changes