[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.~~
> Can these be class methods?

I think I had trouble making them class methods will still keeping them as default arguments in the initializer, but I'll investigate again.

> Is there a clearer name? `defaultBlah`?

Meh, they're meant to be private. I was hoping to use closures for default arguments in the initializer, but that either failed to compile or crashed (I forget).