[WIP] Add a swift only framework.

I ran into #1553 while trying to use ReactiveCocoa in a playground. I thought a framework that only included the new RAC 3.0 Swift API might play better with Playgrounds, and it does.

@jspahrsummers Do you think a swift only ReactiveCocoa framework like this would be generally useful?

I've named the framework `ReactiveSwift` for now, but I'm open to suggestions for a better name.