Accept nil for -subscribe:
Replace subscribeCompleted:^{} with subscribe:nil
-subscribe: can't return nil
Merge pull request #998 from ReactiveCocoa/empty-subscription

Accept nil for -subscribe:
Merge remote-tracking branch 'origin/3.0-development' into bye-bye-actions

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa/RACAction.m
	ReactiveCocoaFramework/ReactiveCocoaTests/RACActionSpec.m