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.

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa/RACQueuedSignalGenerator.h
	ReactiveCocoaFramework/ReactiveCocoa/RACQueuedSignalGenerator.m
	ReactiveCocoaFramework/ReactiveCocoa/RACSignalGenerator+Operations.m
	ReactiveCocoaFramework/ReactiveCocoaTests/RACQueuedSignalGeneratorSpec.m