Add RACQueuedSignalGenerator
Add RACSignalGenerator operator for serialization
Unit tests for RACQueuedSignalGenerator
Replace generator initializers with class constructors

Purely an aesthetic choice, but class constructors look better than
+alloc/-init… when put into a chain of operations.
Merge branch 'queued-generator' into bye-bye-actions
Remove holdover subject in RACQueuedSignalGenerator
Add RACQueuedSignalGenerator.executing