Mac static lib includes

This resolves issues seen with a Mac tool attempting to build against `libReactiveCocoa.a` with some sources that anticipated linking against the framework. Specifically, it adds the four missing headers to the target, and copies all public headers into `include/ReactiveCocoa` instead of `include/ReactiveCocoa-Mac`. This is the same as Mantle’s approach, and allows `#import <ReactiveCocoa/ReactiveCocoa.h>` to succeed with a `HEADER_SEARCH_PATHS` value of `$(BUILT_PRODUCTS_DIR)/include`.