[WIP] Replace commands with simpler alternatives

**:warning: NOT READY TO MERGE :warning:**

As described in https://gist.github.com/jspahrsummers/7000136, I'd love to break down `RACCommand` into a few simpler, more independent pieces.

Of course, `RACCommand` will continue to exist for backwards compatibility, but I'm hoping we can provide enough power in these new primitives to avoid it entirely.

**To do:**

 - [x] `RACSubject`
 - [ ] RefCount
 - [ ] Enabled/disabled operator
 - [x] Kleisli arrows