[WIP] OS X static library

**:warning: NOT READY TO MERGE :warning:**

As we use RAC in more and more things, it'll be handy to embed a complete copy within other frameworks, either to keep its presence hidden (as an implementation detail) or so unused code could hypothetically be stripped out.

The preferred target is still going to be `ReactiveCocoa.framework`. This just adds an alternative for those times when dynamic linking isn't the right choice.