Static signal

I meant to do this as a PR based on #858, but it got merged before I had the free time.

The idea here is unify the primitive classes, `RACReturnSignal`, `RACErrorSignal`, and `RACEmptySignal` into one `RACStaticSignal`. This makes it easy to define similar signals, and this PR adds such methods, a singleton enabled `+never`, and `+defer:`.

@jspahrsummers's original PR used the classes for allocation counting, if that's still desired, then please ignore.