Fix the ReactiveCocoa-Mac target headers

Ensure the public headers includes the libextobjc headers, as the Framework target does
Rename the include directory so that you can `#import "ReactiveCocoa/ReactiveCocoa.h"` whether you’re using the Framework or Static Archive target
Remove private header path override
Merge pull request #983 from ReactiveCocoa/fix-mac-static-archive

Fix the ReactiveCocoa-Mac target headers