[WIP] Break commands down into simpler components

**:warning: NOT READY TO MERGE :warning:**

As described in https://gist.github.com/jspahrsummers/7000136 and https://gist.github.com/jspahrsummers/7080228, and with [very valuable input](https://gist.github.com/jonsterling/7001562) from @jonsterling, I'd love to break down `RACCommand` into a few simpler, more independent pieces.

**To do:**

 - [ ] Deprecate or significantly refactor `RACCommand`
 - [ ] Update mentions of commands in documentation
 - [ ] Refactor `RACCommand` examples
 - [ ] [CHANGELOG](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/3.0-development/CHANGELOG.md)

@paulcbetts