Remove RACUnit

Resolves #1275.

There was one bump here, which is `-rac_liftSelector:withSignals:` guaranteeing a value of `RACUnit` for `void` selectors. Since we're preserving backwards compatibility, and I don't want to rename the method just to solve this, I updated the documentation to a super vague explanation about some kind of “unspecified object.” Behind the scenes, it will remain a `RACUnit`, but callers shouldn't depend on it.