Use existing Copy Headers build phase for iOS

No need for a separate build phase – just set `PUBLIC_HEADERS_FOLDER_PATH` to the place we want to install them. Also updated the iOS demo to not use any explicit search paths, so we can make sure this is actually working correctly.

c.f. github/Mantle#55 and http://www.blog.montgomerie.net/easy-xcode-static-library-subprojects-and-submodules

CC @danielctull