Revert RACAction, use signal generators instead

`RACAction` doesn't make [pure FRP](https://github.com/ReactiveCocoa/ReactiveCocoa/issues/992) any easier. Let's focus on the patterns that do.

Fixes #993. Partly resolves #992 (the other part being #997).

**To do:**

 - [x] Update mentions of `RACAction` in documentation and deprecation messages.
 - [x] Update the CHANGELOG.