[WIP] The Great Swiftening

**:warning: NOT READY TO MERGE :warning:**

Resolves #1365 and #1366. Per my comments in #1369, this is intended to be a long-running branch that will eventually become RAC 3.0, supplanting the [current `3.0-development` work](https://github.com/ReactiveCocoa/ReactiveCocoa/pull/966).

To get us started, I basically just subtree merged [jspahrsummers/RxSwift](https://github.com/jspahrsummers/RxSwift) into here, then bridged several RAC classes back and forth.

~~Currently, only the Mac framework target builds successfully. We may need to revamp our build settings in order to combine Swift and Objective-C correctly.~~
```swift
func ?: <T> (a: Optional<T>, b: T) -> T {
	…
}
```

We can’t use that operator (neither `?` nor `:` are allowed in operator names last I checked) but we could write this as a method on `Optional`, e.g. `e.or(emptyError)`.

Regardless of the details, I’d much rather encapsulate it like that than see us manually unwrap-and-bind everywhere like this.