[WIP] Operator-based backpressure

**:warning: NOT READY TO MERGE :warning:**

I've realized that backpressure can be modeled using [a signal of cold signals](https://gist.github.com/jspahrsummers/5032383). Instead of [tearing apart `RACSignal`](https://github.com/ReactiveCocoa/ReactiveCocoa/pull/808) and putting it back together again, we can just provide some operators to make the signal-of-signals pattern easier and more obvious.

`-flatten:` (the bounded overload of `Merge`, in .NET parlance) already sorta does this, so we only need to customize its behavior when the maximum has been reached. As a bonus, `-concat`, `-switchToLatest` (`Switch`), and `-throttle:` can all be implemented in terms of that operator.

See the changes in `RACSignal+Operations.h` for more info.

**To do:**

 - [ ] Capture https://gist.github.com/jspahrsummers/5032383 in a reusable operator or pattern.
 - [ ] Test the "drop latest" policy.
 - [ ] Test the "drop earliest" policy with N > 1 concurrent signals.
 - [ ] Add some backpressure samples… somewhere. (As tests?)
 - [x] Reimplement `-throttle:` with `-flatten:withPolicy:` and rename it
 - [x] Add a "drop latest" version of `-throttle:`, a la #410 
 - [ ] CHANGELOG

@paulcbetts @erikprice @mattpodwysocki @headinthebox