Copy Mac .a headers into include/ReactiveCocoa.

Previously they were copied into include/ReactiveCocoa-Mac, which meant
that they couldn’t be found by the usual #import <ReactiveCocoa/…>
directive.
Copied Mac .a headers include umbrella & EXT*.

They’re required for projects linking against libReactiveCocoa.a which
desire to use the umbrella header and these other headers.
Merge pull request #1204 from ReactiveCocoa/mac-static-lib-includes

Mac static lib includes